This question tests 2nd grade understanding of adding and subtracting within 100, including using regrouping (carrying and borrowing) when necessary (CCSS 2.NBT.B.5: Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; CCSS 2.NBT.B.7: Add and subtract within 1000, using strategies and algorithms based on place value—focus here on within 100). When adding two-digit numbers, add ones first, then tens. If the ones sum is 10 or more, regroup: write the ones digit and carry 1 ten to the tens column (example: 47 + 28 → $7 + 8 = 15$, write 5 and carry 1 → $4 + 2 + 1 = 7$ → answer is 75). When subtracting, subtract ones first, then tens. If the ones digit in the top number is smaller than the ones digit in the bottom number, regroup (borrow): take 1 ten from the tens column and add 10 to the ones (example: 63 - 27 → can't do 3 - 7, so regroup 6 tens 3 ones into 5 tens 13 ones → $13 - 7 = 6$, $5 - 2 = 3$ → answer is 36). Describe strategies: Place value strategy: Break numbers into tens and ones, add or subtract separately, combine ($47 + 28 = 40 + 20 + 7 + 8 = 60 + 15 = 75$). Number line: Start at first number, jump by tens then ones to add, or jump backward to subtract. In this problem, student must calculate $67 - 19$. To solve, can't do 7 - 9, so regroup 67 to 5 tens 17 ones, subtract $17 - 9 = 8$ and $5 - 1 = 4$, answer 48. Choice B is correct because subtracting $67 - 19$ correctly—regroup to 5 tens 17 ones, $17 - 9 = 8$, $5 - 1 = 4$, giving 48. This correctly applies subtraction with regrouping. Choice C represents a basic fact error ($17 - 9 = 9$ instead of 8, giving 49) or didn't regroup correctly. This error typically happens when students regroup incorrectly or make arithmetic mistakes. To help students: Teach regrouping explicitly with place value understanding. For addition: 'When ones add to 10 or more, we have too many ones. Group 10 ones into 1 ten. Write the remaining ones, carry the 1 ten.' Use base-ten blocks: 4 tens + 7 ones + 2 tens + 8 ones → combine ones ($7 + 8 = 15$ ones = 1 ten + 5 ones), combine tens ($4 + 2 + 1 = 7$ tens) → 75. For subtraction: 'If top ones < bottom ones, we need more ones. Borrow 1 ten (= 10 ones) from tens column.' Show with blocks: 6 tens 3 ones, need to subtract 7 ones (can't), so trade 1 ten for 10 ones (5 tens 13 ones), now can subtract. Practice standard algorithm vertically—align place values, work ones first then tens. Teach number line strategy: to add, jump forward; to subtract, jump backward. For word problems, identify operation: 'got more, combine, total' = add; 'gave away, left, how many more' = subtract. Practice mental math strategies: break apart by tens and ones, use friendly numbers. Watch for: not regrouping when needed, regrouping incorrectly (carrying/borrowing wrong amount), place value errors, wrong operation for context, basic arithmetic mistakes.

This question tests 2nd grade understanding of adding and subtracting within 100, including using regrouping (carrying and borrowing) when necessary (CCSS 2.NBT.B.5: Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; CCSS 2.NBT.B.7: Add and subtract within 1000, using strategies and algorithms based on place value—focus here on within 100). When adding two-digit numbers, add ones first, then tens. If the ones sum is 10 or more, regroup: write the ones digit and carry 1 ten to the tens column (example: 47 + 28 → 7 + 8 = 15, write 5 and carry 1 → 4 + 2 + 1 = 7 → answer is 75). When subtracting, subtract ones first, then tens. If the ones digit in the top number is smaller than the ones digit in the bottom number, regroup (borrow): take 1 ten from the tens column and add 10 to the ones (example: 63 - 27 → can't do 3 - 7, so regroup 6 tens 3 ones into 5 tens 13 ones → 13 - 7 = 6, 5 - 2 = 3 → answer is 36). Describe strategies: Place value strategy: Break numbers into tens and ones, add or subtract separately, combine (47 + 28 = 40 + 20 + 7 + 8 = 60 + 15 = 75). Number line: Start at first number, jump by tens then ones to add, or jump backward to subtract. In this problem, use the number line to add 27 + 36. To solve, start at 27, jump 30 to 57, then 6 to 63, or add ones: 7 + 6 = 13 (write 3, carry 1), tens: 2 + 3 + 1 = 6, answer 63. Choice C is correct because adding 27 + 36 correctly—7 + 6 = 13 (write 3, carry 1), 2 + 3 + 1 = 6, giving 63. This correctly applies addition with regrouping using number line strategy. Choice D represents not carrying when regrouping in addition (7 + 6 = 13 but wrote 53 instead of carrying to get 63, or basic fact error 7 + 6 = 12, giving 62). This error typically happens when students forget to regroup or misjump on the number line. To help students: Teach regrouping explicitly with place value understanding. For addition: 'When ones add to 10 or more, we have too many ones. Group 10 ones into 1 ten. Write the remaining ones, carry the 1 ten.' Use base-ten blocks: 4 tens + 7 ones + 2 tens + 8 ones → combine ones (7 + 8 = 15 ones = 1 ten + 5 ones), combine tens (4 + 2 + 1 = 7 tens) → 75. For subtraction: 'If top ones < bottom ones, we need more ones. Borrow 1 ten (= 10 ones) from tens column.' Show with blocks: 6 tens 3 ones, need to subtract 7 ones (can't), so trade 1 ten for 10 ones (5 tens 13 ones), now can subtract. Practice standard algorithm vertically—align place values, work ones first then tens. Teach number line strategy: to add, jump forward; to subtract, jump backward. For word problems, identify operation: 'got more, combine, total' = add; 'gave away, left, how many more' = subtract. Practice mental math strategies: break apart by tens and ones, use friendly numbers. Watch for: not regrouping when needed, regrouping incorrectly (carrying/borrowing wrong amount), place value errors, wrong operation for context, basic arithmetic mistakes.

This question tests 2nd grade understanding of adding and subtracting within 100, including using regrouping (carrying and borrowing) when necessary (CCSS 2.NBT.B.5: Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; CCSS 2.NBT.B.7: Add and subtract within 1000, using strategies and algorithms based on place value—focus here on within 100). When adding two-digit numbers, add ones first, then tens. If the ones sum is 10 or more, regroup: write the ones digit and carry 1 ten to the tens column (example: 47 + 28 → 7 + 8 = 15, write 5 and carry 1 → 4 + 2 + 1 = 7 → answer is 75). When subtracting, subtract ones first, then tens. If the ones digit in the top number is smaller than the ones digit in the bottom number, regroup (borrow): take 1 ten from the tens column and add 10 to the ones (example: 63 - 27 → can't do 3 - 7, so regroup 6 tens 3 ones into 5 tens 13 ones → 13 - 7 = 6, 5 - 2 = 3 → answer is 36). Place value strategy: Break numbers into tens and ones, add or subtract separately, combine (47 + 28 = 40 + 20 + 7 + 8 = 60 + 15 = 75). Number line: Start at first number, jump by tens then ones to add, or jump backward to subtract. In this problem, the student must calculate 90 - 48. To solve, subtract with regrouping: can't do 0 - 8, so regroup 9 tens 0 ones to 8 tens 10 ones, 10 - 8 = 2, 8 - 4 = 4, answer 42, or use place value: 90 - 40 = 50, 50 - 8 = 42. Choice C is correct because subtracting 90 - 48 correctly—regroup to 8 tens 10 ones, 10 - 8 = 2, 8 - 4 = 4, giving 42. This correctly applies subtraction with regrouping. Choice A represents not regrouping or wrong borrow (perhaps 90 - 48 as 90 - 40 = 50, then 50 + 2 or miscalculated as 52). This error typically happens when students forget to regroup, regroup incorrectly, don't understand borrowing from zero, or make arithmetic mistakes. To help students: Teach regrouping explicitly with place value understanding. For addition: 'When ones add to 10 or more, we have too many ones. Group 10 ones into 1 ten. Write the remaining ones, carry the 1 ten.' Use base-ten blocks: 4 tens + 7 ones + 2 tens + 8 ones → combine ones (7 + 8 = 15 ones = 1 ten + 5 ones), combine tens (4 + 2 + 1 = 7 tens) → 75. For subtraction: 'If top ones < bottom ones, we need more ones. Borrow 1 ten (= 10 ones) from tens column.' Show with blocks: 6 tens 3 ones, need to subtract 7 ones (can't), so trade 1 ten for 10 ones (5 tens 13 ones), now can subtract. Practice standard algorithm vertically—align place values, work ones first then tens. Teach number line strategy: to add, jump forward; to subtract, jump backward. For word problems, identify operation: 'got more, combine, total' = add; 'gave away, left, how many more' = subtract. Practice mental math strategies: break apart by tens and ones, use friendly numbers. Watch for: not regrouping when needed, regrouping incorrectly (carrying/borrowing wrong amount), place value errors, wrong operation for context, basic arithmetic mistakes.

This question tests 2nd grade understanding of adding and subtracting within 100, including using regrouping (carrying and borrowing) when necessary (CCSS 2.NBT.B.5: Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; CCSS 2.NBT.B.7: Add and subtract within 1000, using strategies and algorithms based on place value—focus here on within 100). When adding two-digit numbers, add ones first, then tens. If the ones sum is 10 or more, regroup: write the ones digit and carry 1 ten to the tens column (example: 47 + 28 → 7 + 8 = 15, write 5 and carry 1 → 4 + 2 + 1 = 7 → answer is 75). When subtracting, subtract ones first, then tens. If the ones digit in the top number is smaller than the ones digit in the bottom number, regroup (borrow): take 1 ten from the tens column and add 10 to the ones (example: 63 - 27 → can't do 3 - 7, so regroup 6 tens 3 ones into 5 tens 13 ones → 13 - 7 = 6, 5 - 2 = 3 → answer is 36). Place value strategy: Break numbers into tens and ones, add or subtract separately, combine (47 + 28 = 40 + 20 + 7 + 8 = 60 + 15 = 75). Number line: Start at first number, jump by tens then ones to add, or jump backward to subtract. In this problem, there were 78 students and 26 went home, a word problem requiring subtraction to find how many are left. To solve, subtract 78 - 26: ones 8 - 6 = 2 (no borrow needed), tens 7 - 2 = 5, answer 52, or use place value: 70 - 20 = 50, 8 - 6 = 2, total 52. Choice C is correct because subtracting 78 - 26 correctly—8 - 6 = 2, 7 - 2 = 5, giving 52, or in context: 78 total, 26 left, 52 remain. This correctly applies subtraction without regrouping to the word problem. Choice B represents a wrong operation (perhaps added 78 + 26 = 104 but that's not an option, or miscalculated as 62 by borrowing unnecessarily or arithmetic error). This error typically happens when students confuse operations (add instead of subtract), make arithmetic mistakes, or don't understand word problem keywords like 'left'. To help students: Teach regrouping explicitly with place value understanding. For addition: 'When ones add to 10 or more, we have too many ones. Group 10 ones into 1 ten. Write the remaining ones, carry the 1 ten.' Use base-ten blocks: 4 tens + 7 ones + 2 tens + 8 ones → combine ones (7 + 8 = 15 ones = 1 ten + 5 ones), combine tens (4 + 2 + 1 = 7 tens) → 75. For subtraction: 'If top ones < bottom ones, we need more ones. Borrow 1 ten (= 10 ones) from tens column.' Show with blocks: 6 tens 3 ones, need to subtract 7 ones (can't), so trade 1 ten for 10 ones (5 tens 13 ones), now can subtract. Practice standard algorithm vertically—align place values, work ones first then tens. Teach number line strategy: to add, jump forward; to subtract, jump backward. For word problems, identify operation: 'got more, combine, total' = add; 'gave away, left, how many more' = subtract. Practice mental math strategies: break apart by tens and ones, use friendly numbers. Watch for: not regrouping when needed, regrouping incorrectly (carrying/borrowing wrong amount), place value errors, wrong operation for context, basic arithmetic mistakes.

This question tests 2nd grade understanding of adding and subtracting within 100, including using regrouping (carrying and borrowing) when necessary (CCSS 2.NBT.B.5: Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; CCSS 2.NBT.B.7: Add and subtract within 1000, using strategies and algorithms based on place value—focus here on within 100). When adding two-digit numbers, add ones first, then tens. If the ones sum is 10 or more, regroup: write the ones digit and carry 1 ten to the tens column (example: 47 + 28 → 7 + 8 = 15, write 5 and carry 1 → 4 + 2 + 1 = 7 → answer is 75). When subtracting, subtract ones first, then tens. If the ones digit in the top number is smaller than the ones digit in the bottom number, regroup (borrow): take 1 ten from the tens column and add 10 to the ones (example: 63 - 27 → can't do 3 - 7, so regroup 6 tens 3 ones into 5 tens 13 ones → 13 - 7 = 6, 5 - 2 = 3 → answer is 36). Describe strategies: Place value strategy: Break numbers into tens and ones, add or subtract separately, combine (47 + 28 = 40 + 20 + 7 + 8 = 60 + 15 = 75). Number line: Start at first number, jump by tens then ones to add, or jump backward to subtract. In this problem, the student must add 24 + 35 to find the total. To solve, add ones: 4 + 5 = 9 (no carry), add tens: 2 + 3 = 5, answer 59. Choice C is correct because adding 24 + 35 correctly—4 + 5 = 9, 2 + 3 = 5, giving 59. This correctly applies addition without regrouping. Choice A represents a specific error: carried unnecessarily (4 + 5 = 9 but perhaps thought it was 9 + carry to get 69). This error typically happens when students regroup incorrectly, make arithmetic mistakes, or confuse place values. To help students: Teach regrouping explicitly with place value understanding. For addition: 'When ones add to 10 or more, we have too many ones. Group 10 ones into 1 ten. Write the remaining ones, carry the 1 ten.' Use base-ten blocks: 4 tens + 7 ones + 2 tens + 8 ones → combine ones (7 + 8 = 15 ones = 1 ten + 5 ones), combine tens (4 + 2 + 1 = 7 tens) → 75. For subtraction: 'If top ones < bottom ones, we need more ones. Borrow 1 ten (= 10 ones) from tens column.' Show with blocks: 6 tens 3 ones, need to subtract 7 ones (can't), so trade 1 ten for 10 ones (5 tens 13 ones), now can subtract. Practice standard algorithm vertically—align place values, work ones first then tens. Teach number line strategy: to add, jump forward; to subtract, jump backward. For word problems, identify operation: 'got more, combine, total' = add; 'gave away, left, how many more' = subtract. Practice mental math strategies: break apart by tens and ones, use friendly numbers. Watch for: not regrouping when needed, regrouping incorrectly (carrying/borrowing wrong amount), place value errors, wrong operation for context, basic arithmetic mistakes.

This question tests 2nd grade understanding of adding and subtracting within 100, including using regrouping (carrying and borrowing) when necessary (CCSS 2.NBT.B.5: Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; CCSS 2.NBT.B.7: Add and subtract within 1000, using strategies and algorithms based on place value—focus here on within 100). When adding two-digit numbers, add ones first, then tens. If the ones sum is 10 or more, regroup: write the ones digit and carry 1 ten to the tens column (example: 47 + 28 → 7 + 8 = 15, write 5 and carry 1 → 4 + 2 + 1 = 7 → answer is 75). When subtracting, subtract ones first, then tens. If the ones digit in the top number is smaller than the ones digit in the bottom number, regroup (borrow): take 1 ten from the tens column and add 10 to the ones (example: 63 - 27 → can't do 3 - 7, so regroup 6 tens 3 ones into 5 tens 13 ones → 13 - 7 = 6, 5 - 2 = 3 → answer is 36). Describe strategies: Place value strategy: Break numbers into tens and ones, add or subtract separately, combine (47 + 28 = 40 + 20 + 7 + 8 = 60 + 15 = 75). Number line: Start at first number, jump by tens then ones to add, or jump backward to subtract. In this problem, student must add 46 + 27 using place value. To solve, break into tens and ones: (40 + 20) + (6 + 7) = 60 + 13 = 73, or add ones: 6 + 7 = 13 (write 3, carry 1), add tens: 4 + 2 + 1 = 7, answer 73. Choice B is correct because adding 46 + 27 correctly—6 + 7 = 13 (write 3, carry 1), 4 + 2 + 1 = 7, giving 73. This correctly applies addition with regrouping. Choice D represents not carrying when regrouping in addition (6 + 7 = 13 but wrote 63 instead of carrying to get 73, or basic fact error 6 + 7 = 12, giving 72). This error typically happens when students forget to regroup or make arithmetic mistakes. To help students: Teach regrouping explicitly with place value understanding. For addition: 'When ones add to 10 or more, we have too many ones. Group 10 ones into 1 ten. Write the remaining ones, carry the 1 ten.' Use base-ten blocks: 4 tens + 7 ones + 2 tens + 8 ones → combine ones (7 + 8 = 15 ones = 1 ten + 5 ones), combine tens (4 + 2 + 1 = 7 tens) → 75. For subtraction: 'If top ones < bottom ones, we need more ones. Borrow 1 ten (= 10 ones) from tens column.' Show with blocks: 6 tens 3 ones, need to subtract 7 ones (can't), so trade 1 ten for 10 ones (5 tens 13 ones), now can subtract. Practice standard algorithm vertically—align place values, work ones first then tens. Teach number line strategy: to add, jump forward; to subtract, jump backward. For word problems, identify operation: 'got more, combine, total' = add; 'gave away, left, how many more' = subtract. Practice mental math strategies: break apart by tens and ones, use friendly numbers. Watch for: not regrouping when needed, regrouping incorrectly (carrying/borrowing wrong amount), place value errors, wrong operation for context, basic arithmetic mistakes.

This question tests 2nd grade understanding of adding and subtracting within 100, including using regrouping (carrying and borrowing) when necessary (CCSS 2.NBT.B.5: Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; CCSS 2.NBT.B.7: Add and subtract within 1000, using strategies and algorithms based on place value—focus here on within 100). When adding two-digit numbers, add ones first, then tens. If the ones sum is 10 or more, regroup: write the ones digit and carry 1 ten to the tens column (example: $47 + 28$ → $7 + 8 = 15$, write 5 and carry 1 → $4 + 2 + 1 = 7$ → answer is 75). When subtracting, subtract ones first, then tens. If the ones digit in the top number is smaller than the ones digit in the bottom number, regroup (borrow): take 1 ten from the tens column and add 10 to the ones (example: $63 - 27$ → can't do 3 - 7, so regroup 6 tens 3 ones into 5 tens 13 ones → $13 - 7 = 6$, $5 - 2 = 3$ → answer is 36). Place value strategy: Break numbers into tens and ones, add or subtract separately, combine ($47 + 28 = 40 + 20 + 7 + 8 = 60 + 15 = 75$). Number line: Start at first number, jump by tens then ones to add, or jump backward to subtract. In this problem, Maya has 52 beads and gets 19 more, a word problem requiring addition. To solve, add $52 + 19$: ones $2 + 9 = 11$ (write 1, carry 1), tens $5 + 1 + 1 = 7$, answer 71, or use place value: $50 + 10 = 60$, $2 + 9 = 11$, total 71. Choice B is correct because adding $52 + 19$ correctly—$2 + 9 = 11$ (write 1, carry 1), $5 + 1 + 1 = 7$, giving 71, or in context: Maya had 52, got 19 more, total 71. This correctly applies addition with regrouping to the word problem. Choice A represents not carrying in addition ($5 + 1 = 6$, $2 + 9 = 11$ but wrote 61 without proper carry or misadded). This error typically happens when students forget to regroup, make arithmetic mistakes, or confuse the operation in the word problem. To help students: Teach regrouping explicitly with place value understanding. For addition: 'When ones add to 10 or more, we have too many ones. Group 10 ones into 1 ten. Write the remaining ones, carry the 1 ten.' Use base-ten blocks: 4 tens + 7 ones + 2 tens + 8 ones → combine ones ($7 + 8 = 15$ ones = 1 ten + 5 ones), combine tens ($4 + 2 + 1 = 7$ tens) → 75. For subtraction: 'If top ones < bottom ones, we need more ones. Borrow 1 ten (= 10 ones) from tens column.' Show with blocks: 6 tens 3 ones, need to subtract 7 ones (can't), so trade 1 ten for 10 ones (5 tens 13 ones), now can subtract. Practice standard algorithm vertically—align place values, work ones first then tens. Teach number line strategy: to add, jump forward; to subtract, jump backward. For word problems, identify operation: 'got more, combine, total' = add; 'gave away, left, how many more' = subtract. Practice mental math strategies: break apart by tens and ones, use friendly numbers. Watch for: not regrouping when needed, regrouping incorrectly (carrying/borrowing wrong amount), place value errors, wrong operation for context, basic arithmetic mistakes.

This question tests 2nd grade understanding of adding and subtracting within 100, including using regrouping (carrying and borrowing) when necessary (CCSS 2.NBT.B.5: Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; CCSS 2.NBT.B.7: Add and subtract within 1000, using strategies and algorithms based on place value—focus here on within 100). When adding two-digit numbers, add ones first, then tens. If the ones sum is 10 or more, regroup: write the ones digit and carry 1 ten to the tens column (example: 47 + 28 → 7 + 8 = 15, write 5 and carry 1 → 4 + 2 + 1 = 7 → answer is 75). When subtracting, subtract ones first, then tens. If the ones digit in the top number is smaller than the ones digit in the bottom number, regroup (borrow): take 1 ten from the tens column and add 10 to the ones (example: 63 - 27 → can't do 3 - 7, so regroup 63 to 5 tens 13 ones → 13 - 7 = 6, 5 - 2 = 3 → answer is 36). Describe strategies: Place value strategy: Break numbers into tens and ones, add or subtract separately, combine (47 + 28 = 40 + 20 + 7 + 8 = 60 + 15 = 75). Number line: Start at first number, jump by tens then ones to add, or jump backward to subtract. In this problem, the student must add 56 + 18 using a number line. To solve, start at 56, jump +10 to 66, then +8 to 74, or jump +18 directly for the sum 74. Choice C is correct because adding 56 + 18 correctly—6 + 8 = 14 (write 4, carry 1), 5 + 1 + 1 = 7, giving 74. This correctly applies addition with regrouping. Choice A represents not carrying properly (perhaps 56 + 8 = 64 without adding the full 18). This error typically happens when students forget to regroup, make arithmetic mistakes, or miscount jumps on the number line. To help students: Teach regrouping explicitly with place value understanding. For addition: 'When ones add to 10 or more, we have too many ones. Group 10 ones into 1 ten. Write the remaining ones, carry the 1 ten.' Use base-ten blocks: 4 tens + 7 ones + 2 tens + 8 ones → combine ones (7 + 8 = 15 ones = 1 ten + 5 ones), combine tens (4 + 2 + 1 = 7 tens) → 75. For subtraction: 'If top ones < bottom ones, we need more ones. Borrow 1 ten (= 10 ones) from tens column.' Show with blocks: 6 tens 3 ones, need to subtract 7 ones (can't), so trade 1 ten for 10 ones (5 tens 13 ones), now can subtract. Practice standard algorithm vertically—align place values, work ones first then tens. Teach number line strategy: to add, jump forward; to subtract, jump backward. For word problems, identify operation: 'got more, combine, total' = add; 'gave away, left, how many more' = subtract. Practice mental math strategies: break apart by tens and ones, use friendly numbers. Watch for: not regrouping when needed, regrouping incorrectly (carrying/borrowing wrong amount), place value errors, wrong operation for context, basic arithmetic mistakes.

This question tests 2nd grade understanding of adding and subtracting within 100, including using regrouping (carrying and borrowing) when necessary (CCSS 2.NBT.B.5: Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; CCSS 2.NBT.B.7: Add and subtract within 1000, using strategies and algorithms based on place value—focus here on within 100). When adding two-digit numbers, add ones first, then tens. If the ones sum is 10 or more, regroup: write the ones digit and carry 1 ten to the tens column (example: 47 + 28 → 7 + 8 = 15, write 5 and carry 1 → 4 + 2 + 1 = 7 → answer is 75). When subtracting, subtract ones first, then tens. If the ones digit in the top number is smaller than the ones digit in the bottom number, regroup (borrow): take 1 ten from the tens column and add 10 to the ones (example: 63 - 27 → can't do 3 - 7, so regroup 6 tens 3 ones into 5 tens 13 ones → 13 - 7 = 6, 5 - 2 = 3 → answer is 36). Place value strategy: Break numbers into tens and ones, add or subtract separately, combine (47 + 28 = 40 + 20 + 7 + 8 = 60 + 15 = 75). Number line: Start at first number, jump by tens then ones to add, or jump backward to subtract. In this problem, Keisha had 64 stickers and gave away 29, a word problem requiring subtraction to find how many left. To solve, subtract 64 - 29: can't do 4 - 9, regroup 6 tens 4 ones to 5 tens 14 ones, 14 - 9 = 5, 5 - 2 = 3, answer 35, or use place value: 60 - 20 = 40, but adjust for ones (4 - 9 requires borrow, 40 - 5 = 35 after accounting). Choice B is correct because subtracting 64 - 29 correctly—regroup to 5 tens 14 ones, 14 - 9 = 5, 5 - 2 = 3, giving 35, or in context: 64 total, gave away 29, 35 left. This correctly applies subtraction with regrouping to the word problem. Choice A represents not regrouping or arithmetic error (perhaps 64 - 19 = 45 or forgot to borrow properly). This error typically happens when students forget to regroup, make arithmetic mistakes, or confuse the operation in the word problem. To help students: Teach regrouping explicitly with place value understanding. For addition: 'When ones add to 10 or more, we have too many ones. Group 10 ones into 1 ten. Write the remaining ones, carry the 1 ten.' Use base-ten blocks: 4 tens + 7 ones + 2 tens + 8 ones → combine ones (7 + 8 = 15 ones = 1 ten + 5 ones), combine tens (4 + 2 + 1 = 7 tens) → 75. For subtraction: 'If top ones < bottom ones, we need more ones. Borrow 1 ten (= 10 ones) from tens column.' Show with blocks: 6 tens 3 ones, need to subtract 7 ones (can't), so trade 1 ten for 10 ones (5 tens 13 ones), now can subtract. Practice standard algorithm vertically—align place values, work ones first then tens. Teach number line strategy: to add, jump forward; to subtract, jump backward. For word problems, identify operation: 'got more, combine, total' = add; 'gave away, left, how many more' = subtract. Practice mental math strategies: break apart by tens and ones, use friendly numbers. Watch for: not regrouping when needed, regrouping incorrectly (carrying/borrowing wrong amount), place value errors, wrong operation for context, basic arithmetic mistakes.

This question tests 2nd grade understanding of adding and subtracting within 100, including using regrouping (carrying and borrowing) when necessary (CCSS 2.NBT.B.5: Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; CCSS 2.NBT.B.7: Add and subtract within 1000, using strategies and algorithms based on place value—focus here on within 100). When adding two-digit numbers, add ones first, then tens. If the ones sum is 10 or more, regroup: write the ones digit and carry 1 ten to the tens column (example: 47 + 28 → 7 + 8 = 15, write 5 and carry 1 → 4 + 2 + 1 = 7 → answer is 75). When subtracting, subtract ones first, then tens. If the ones digit in the top number is smaller than the ones digit in the bottom number, regroup (borrow): take 1 ten from the tens column and add 10 to the ones (example: 63 - 27 → can't do 3 - 7, so regroup 6 tens 3 ones into 5 tens 13 ones → 13 - 7 = 6, 5 - 2 = 3 → answer is 36). Describe strategies: Place value strategy: Break numbers into tens and ones, add or subtract separately, combine (47 + 28 = 40 + 20 + 7 + 8 = 60 + 15 = 75). Number line: Start at first number, jump by tens then ones to add, or jump backward to subtract. In this problem, the student must add 36 + 29 using a number line to find the sum. To solve, start at 36 on the number line, jump 20 to 56, then jump 9 to 65; or use place value: 30 + 20 = 50, 6 + 9 = 15, 50 + 15 = 65. Choice B is correct because adding 36 + 29 correctly—6 + 9 = 15 (write 5, carry 1), 3 + 2 + 1 = 6, giving 65. This correctly applies addition with regrouping. Choice C represents a specific error: basic fact error (6 + 9 = 15 but perhaps added without carrying properly or miscounted on number line to 75). This error typically happens when students make arithmetic mistakes, forget to regroup, or miscount jumps on the number line. To help students: Teach regrouping explicitly with place value understanding. For addition: 'When ones add to 10 or more, we have too many ones. Group 10 ones into 1 ten. Write the remaining ones, carry the 1 ten.' Use base-ten blocks: 4 tens + 7 ones + 2 tens + 8 ones → combine ones (7 + 8 = 15 ones = 1 ten + 5 ones), combine tens (4 + 2 + 1 = 7 tens) → 75. For subtraction: 'If top ones < bottom ones, we need more ones. Borrow 1 ten (= 10 ones) from tens column.' Show with blocks: 6 tens 3 ones, need to subtract 7 ones (can't), so trade 1 ten for 10 ones (5 tens 13 ones), now can subtract. Practice standard algorithm vertically—align place values, work ones first then tens. Teach number line strategy: to add, jump forward; to subtract, jump backward. For word problems, identify operation: 'got more, combine, total' = add; 'gave away, left, how many more' = subtract. Practice mental math strategies: break apart by tens and ones, use friendly numbers. Watch for: not regrouping when needed, regrouping incorrectly (carrying/borrowing wrong amount), place value errors, wrong operation for context, basic arithmetic mistakes.