This question tests 2nd grade understanding of mentally adding and subtracting 10 or 100 to/from three-digit numbers using place value patterns (CCSS 2.NBT.B.8: Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900). To mentally add or subtract 10 or 100, understand which place value digit changes: (1) Adding 10 means adding 1 ten, so tens digit increases by 1 (376 + 10: 7 tens becomes 8 tens = 386; ones and hundreds stay same). (2) Subtracting 10 means subtracting 1 ten, so tens digit decreases by 1 (376 - 10: 7 tens becomes 6 tens = 366). (3) Adding 100 means adding 1 hundred, so hundreds digit increases by 1 (376 + 100: 3 hundreds becomes 4 hundreds = 476; tens and ones stay same). (4) Subtracting 100 means subtracting 1 hundred, so hundreds digit decreases by 1 (528 - 100: 5 hundreds becomes 4 hundreds = 428). Describe pattern: Look only at the digit that changes—just add or subtract 1 from that digit, leave other digits alone. This makes calculation easy to do mentally without writing. In this problem, student must continue pattern of subtracting 10: 780, 770, 760, ___. To solve mentally, identify operation (-10 each time), identify which digit changes (tens for -10), change that digit by 1 (decrease for subtraction), keep other digits same: 760 - 10 → tens 6→5, answer 750. Choice A is correct because pattern 780, 770, 760 decreases by 10 each time (tens digit down by 1), so next is 750. This correctly identifies and changes only the appropriate place value digit. Choice B represents specific error: subtracted 100 instead of 10, giving 660 instead of 750. This error typically happens when students confuse subtracting 10 with subtracting 100 or don't recognize the pattern of changing tens. To help students: Teach place value explicitly—'Subtracting 10 means subtracting 1 ten. Which place is tens? Middle digit. So middle digit goes down by 1. Hundreds and ones stay same.' Use place value chart: write 760, show subtracting 10 changes only tens: 7 hundreds | 6 tens → 5 tens | 0 ones = 750. Practice with hundreds: 'Subtracting 100 means subtracting 1 hundred. Which place? Leftmost digit. So leftmost digit goes down by 1. Tens and ones stay same.' Show patterns visually: 780, 770, 760, 750—notice only tens digit changes (8→7→6→5), everything else same. Use number line: jump by 10s backward, notice pattern. Connect to skip counting: counting backward by 10s (780, 770, 760...) uses same skill. Practice mental calculation: 'Don't write. Just change one digit in your head.' Teach exceptions: 398 + 10 requires regrouping (9 tens + 1 ten = 10 tens = 1 hundred 0 tens), 302 - 10 requires borrowing (0 tens, borrow from hundreds). Watch for: changing wrong digit, changing wrong amount, confusing ±10 with ±100, not understanding place value, changing multiple digits.

This question tests 2nd grade understanding of mentally adding and subtracting 10 or 100 to/from three-digit numbers using place value patterns (CCSS 2.NBT.B.8: Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900). To mentally add or subtract 10 or 100, understand which place value digit changes: (1) Adding 10 means adding 1 ten, so tens digit increases by 1 (376 + 10: 7 tens becomes 8 tens = 386; ones and hundreds stay same). (2) Subtracting 10 means subtracting 1 ten, so tens digit decreases by 1 (376 - 10: 7 tens becomes 6 tens = 366). (3) Adding 100 means adding 1 hundred, so hundreds digit increases by 1 (376 + 100: 3 hundreds becomes 4 hundreds = 476; tens and ones stay same). (4) Subtracting 100 means subtracting 1 hundred, so hundreds digit decreases by 1 (528 - 100: 5 hundreds becomes 4 hundreds = 428). In this problem, the student must subtract 100 from 639 mentally. To solve mentally, identify operation (-100), identify which digit changes (hundreds for -100), change that digit by 1 (decrease for subtraction), keep other digits same: 639 - 100 → hundreds 6→5, answer 539. Choice A is correct because subtracting 100 from 639 means subtracting 1 from hundreds digit (6→5), keeping tens (3) and ones (9) same, giving 539. This correctly identifies and changes only the appropriate place value digit. Choice D represents a specific error: subtracted 10 instead of 100 (639-10=629), giving 629 instead of 539. This error typically happens when students confuse -10 with -100, or don't understand which place value changes with -100 vs -10. To help students: Teach place value explicitly—'Subtracting 100 means subtracting 1 hundred. Which place is hundreds? Leftmost digit. So leftmost digit goes down by 1. Tens and ones stay same.' Use place value chart: write 639, show subtracting 100 changes only hundreds: 6 hundreds → 5 hundreds | 3 tens | 9 ones = 539. Practice with tens: 'Subtracting 10 means subtracting 1 ten. Which place? Middle digit. So middle digit goes down by 1. Hundreds and ones stay same.' Show patterns visually: 900, 800, 700, 600—notice only hundreds digit changes (9→8→7→6), everything else same. Use number line: jump back by 10s or 100s, notice pattern. Connect to skip counting: counting backward by 10s (340, 330, 320...) or by 100s (400, 300, 200...) uses same skill. Practice mental calculation: 'Don't write. Just change one digit in your head.' Teach exceptions: 302 - 10 requires borrowing (0 tens, borrow from hundreds). Watch for: changing wrong digit, changing wrong amount, confusing -10 with -100, not understanding place value, changing multiple digits.

This question tests 2nd grade understanding of mentally adding and subtracting 10 or 100 to/from three-digit numbers using place value patterns (CCSS 2.NBT.B.8: Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900). To mentally add or subtract 10 or 100, understand which place value digit changes: (1) Adding 10 means adding 1 ten, so tens digit increases by 1 (376 + 10: 7 tens becomes 8 tens = 386; ones and hundreds stay same). (2) Subtracting 10 means subtracting 1 ten, so tens digit decreases by 1 (376 - 10: 7 tens becomes 6 tens = 366). (3) Adding 100 means adding 1 hundred, so hundreds digit increases by 1 (376 + 100: 3 hundreds becomes 4 hundreds = 476; tens and ones stay same). (4) Subtracting 100 means subtracting 1 hundred, so hundreds digit decreases by 1 (528 - 100: 5 hundreds becomes 4 hundreds = 428). In this problem, when you add 10 to 534, identify which digit changes. To solve mentally, identify operation (+10), identify which digit changes (tens for +10): 534 +10=544, tens 3→4, others same. Choice D is correct because adding 10 to 534 changes the tens digit (3→4), as it's adding 1 ten. This correctly identifies the appropriate place value digit. Choice A represents a specific error: said ones digit changes, when ones changes for +1, not +10. This error typically happens when students don't understand which place value changes with +10 vs +1, confuse tens and ones. To help students: Teach place value explicitly—'Adding 10 means adding 1 ten. Which place is tens? Middle digit. So middle digit changes.' Use place value chart: write 534, show adding 10 changes only tens: 5 hundreds | 3 tens → 4 tens | 4 ones = 544. Practice with hundreds: 'Adding 100 changes hundreds digit.' Show patterns visually: 534 +10=544, notice only tens digit changes (3→4). Use number line: jump by 10, see tens change. Connect to skip counting: counting by 10s changes tens digit. Practice mental calculation: 'Don't write. Just think which digit changes.' Teach exceptions: 399 +10=409, tens 9→0 and hundreds 3→4 due to regrouping. Watch for: saying wrong digit, confusing +10 with +100, not understanding place value.

This question tests 2nd grade understanding of mentally adding and subtracting 10 or 100 to/from three-digit numbers using place value patterns (CCSS 2.NBT.B.8: Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900). To mentally add or subtract 10 or 100, understand which place value digit changes: (1) Adding 10 means adding 1 ten, so tens digit increases by 1 ($376 + 10$: 7 tens becomes 8 tens = 386; ones and hundreds stay same). (2) Subtracting 10 means subtracting 1 ten, so tens digit decreases by 1 ($376 - 10$: 7 tens becomes 6 tens = 366). (3) Adding 100 means adding 1 hundred, so hundreds digit increases by 1 ($376 + 100$: 3 hundreds becomes 4 hundreds = 476; tens and ones stay same). (4) Subtracting 100 means subtracting 1 hundred, so hundreds digit decreases by 1 ($528 - 100$: 5 hundreds becomes 4 hundreds = 428). Look only at the digit that changes—just add or subtract 1 from that digit, leave other digits alone. This makes calculation easy to do mentally without writing. In this problem, the student must add 100 to 326 mentally in a word problem context. To solve mentally, identify operation (+100), identify which digit changes (hundreds for +100), change that digit by 1 (increase for addition), keep other digits same: $326 + 100$ → hundreds 3→4, answer 426. Choice A is correct because adding 100 to 326 means adding 1 to hundreds digit (3→4), keeping tens (2) and ones (6) same, giving 426. This correctly identifies and changes only the appropriate place value digit. Choice B represents a specific error: added 10 instead of 100 (increased tens digit by 1, $326+10=336$ instead of 426). This error typically happens when students confuse +10 with +100, don't understand which place value changes, or add to the wrong digit. To help students: Teach place value explicitly—'Adding 100 means adding 1 hundred. Which place? Leftmost digit. So leftmost digit goes up by 1. Tens and ones stay same.' Use place value chart: write 326, show adding 100 changes only hundreds: 3 hundreds → 4 hundreds | 2 tens | 6 ones = 426. Practice with tens: 'Adding 10 means adding 1 ten. Which place? Middle digit. So middle digit goes up by 1. Hundreds and ones stay same.' Show patterns visually: 320, 330, 340, 350—notice only tens digit changes (2→3→4→5), everything else same. Use number line: jump by 10s or 100s, notice pattern. Connect to skip counting: counting by 10s (320, 330, 340...) or by 100s (200, 300, 400...) uses same skill. Practice mental calculation: 'Don't write. Just change one digit in your head.' Teach exceptions: 398 + 10 requires regrouping (9 tens + 1 ten = 10 tens = 1 hundred 0 tens), 302 - 10 requires borrowing (0 tens, borrow from hundreds). Watch for: changing wrong digit, changing wrong amount, confusing +10 with +100, not understanding place value, changing multiple digits.

This question tests 2nd grade understanding of mentally adding and subtracting 10 or 100 to/from three-digit numbers using place value patterns (CCSS 2.NBT.B.8: Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900). To mentally add or subtract 10 or 100, understand which place value digit changes: (1) Adding 10 means adding 1 ten, so tens digit increases by 1 (376 + 10: 7 tens becomes 8 tens = 386; ones and hundreds stay same). (2) Subtracting 10 means subtracting 1 ten, so tens digit decreases by 1 (376 - 10: 7 tens becomes 6 tens = 366). (3) Adding 100 means adding 1 hundred, so hundreds digit increases by 1 (376 + 100: 3 hundreds becomes 4 hundreds = 476; tens and ones stay same). (4) Subtracting 100 means subtracting 1 hundred, so hundreds digit decreases by 1 (528 - 100: 5 hundreds becomes 4 hundreds = 428). Look only at the digit that changes—just add or subtract 1 from that digit, leave other digits alone. This makes calculation easy to do mentally without writing. In this problem, the student must continue the pattern of adding 10: 240, 250, 260, ___. To solve mentally, identify operation (+10 each time), identify which digit changes (tens for +10), change that digit by 1 (increase for addition), keep other digits same: from 260 + 10 → tens 6→7, answer 270. Choice C is correct because the pattern 240, 250, 260 increases by 10 each time (tens digit up by 1), so next is 270. This correctly identifies and changes only the appropriate place value digit. Choice A represents a specific error: added 100 instead of 10 (increased hundreds digit by 1, giving 360 instead of 270). This error typically happens when students confuse +10 with +100, don't recognize the pattern of changing tens digit, or add to the wrong digit. To help students: Teach place value explicitly—'Adding 10 means adding 1 ten. Which place is tens? Middle digit. So middle digit goes up by 1. Hundreds and ones stay same.' Use place value chart: write 260, show adding 10 changes only tens: 2 hundreds | 6 tens → 7 tens | 0 ones = 270. Practice with hundreds: 'Adding 100 means adding 1 hundred. Which place? Leftmost digit. So leftmost digit goes up by 1. Tens and ones stay same.' Show patterns visually: 320, 330, 340, 350—notice only tens digit changes (2→3→4→5), everything else same. Use number line: jump by 10s or 100s, notice pattern. Connect to skip counting: counting by 10s (320, 330, 340...) or by 100s (200, 300, 400...) uses same skill. Practice mental calculation: 'Don't write. Just change one digit in your head.' Teach exceptions: 398 + 10 requires regrouping (9 tens + 1 ten = 10 tens = 1 hundred 0 tens), 302 - 10 requires borrowing (0 tens, borrow from hundreds). Watch for: changing wrong digit, changing wrong amount, confusing +10 with +100, not understanding place value, changing multiple digits.

This question tests 2nd grade understanding of mentally adding and subtracting 10 or 100 to/from three-digit numbers using place value patterns (CCSS 2.NBT.B.8: Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900). To mentally add or subtract 10 or 100, understand which place value digit changes: (1) Adding 10 means adding 1 ten, so tens digit increases by 1 ($376 + 10$: 7 tens becomes 8 tens = $386$; ones and hundreds stay same). (2) Subtracting 10 means subtracting 1 ten, so tens digit decreases by 1 ($376 - 10$: 7 tens becomes 6 tens = $366$). (3) Adding 100 means adding 1 hundred, so hundreds digit increases by 1 ($376 + 100$: 3 hundreds becomes 4 hundreds = $476$; tens and ones stay same). (4) Subtracting 100 means subtracting 1 hundred, so hundreds digit decreases by 1 ($528 - 100$: 5 hundreds becomes 4 hundreds = $428$). Look only at the digit that changes—just add or subtract 1 from that digit, leave other digits alone. This makes calculation easy to do mentally without writing. In this problem, the student must subtract 10 from $905$ mentally. To solve mentally, identify operation (-10), identify which digit changes (tens for -10), change that digit by 1 (decrease for subtraction), keep other digits same: $905 - 10$ → tens 0→9 (but since 0 requires borrowing: 9 hundreds 0 tens 5 ones becomes 8 hundreds 9 tens 5 ones), answer $895$. Choice B is correct because subtracting 10 from $905$ means subtracting 1 from tens digit (0-1 requires borrowing, so hundreds 9→8, tens 0→9), keeping ones (5) same, giving $895$. This correctly identifies and changes only the appropriate place value digit. Choice C represents a specific error: subtracted 1 from ones instead of tens ($905-1=904$ instead of $895$). This error typically happens when students don't understand which place value changes with -10 vs -1, confuse tens and ones, or subtract from the wrong digit. To help students: Teach place value explicitly—'Subtracting 10 means subtracting 1 ten. Which place is tens? Middle digit. So middle digit goes down by 1. Hundreds and ones stay same.' Use place value chart: write $905$, show subtracting 10 changes only tens with borrowing: 9 hundreds | 0 tens → borrow to 8 hundreds | 10 tens -1=9 tens | 5 ones = $895$. Practice with hundreds: 'Subtracting 100 means subtracting 1 hundred. Which place? Leftmost digit. So leftmost digit goes down by 1. Tens and ones stay same.' Show patterns visually: $320$, $330$, $340$, $350$—notice only tens digit changes (2→3→4→5), everything else same. Use number line: jump by 10s or 100s, notice pattern. Connect to skip counting: counting by 10s ($320$, $330$, $340$...) or by 100s ($200$, $300$, $400$...) uses same skill. Practice mental calculation: 'Don't write. Just change one digit in your head.' Teach exceptions: $398 + 10$ requires regrouping (9 tens + 1 ten = 10 tens = 1 hundred 0 tens), $302 - 10$ requires borrowing (0 tens, borrow from hundreds). Watch for: changing wrong digit, changing wrong amount, confusing -10 with -100, not understanding place value, changing multiple digits.

This question tests 2nd grade understanding of mentally adding and subtracting 10 or 100 to/from three-digit numbers using place value patterns (CCSS 2.NBT.B.8: Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900). To mentally add or subtract 10 or 100, understand which place value digit changes: (1) Adding 10 means adding 1 ten, so tens digit increases by 1 (376 + 10: 7 tens becomes 8 tens = 386; ones and hundreds stay same). (2) Subtracting 10 means subtracting 1 ten, so tens digit decreases by 1 (376 - 10: 7 tens becomes 6 tens = 366). (3) Adding 100 means adding 1 hundred, so hundreds digit increases by 1 (376 + 100: 3 hundreds becomes 4 hundreds = 476; tens and ones stay same). (4) Subtracting 100 means subtracting 1 hundred, so hundreds digit decreases by 1 (528 - 100: 5 hundreds becomes 4 hundreds = 428). Describe pattern: Look only at the digit that changes—just add or subtract 1 from that digit, leave other digits alone. This makes calculation easy to do mentally without writing. In this problem, student must identify which digit changes when adding 100 to 431. To solve mentally, identify operation (+100), identify which digit changes (hundreds for +100), tens and ones stay same: 431 + 100 → hundreds 4→5, tens 3 same, ones 1 same = 531. Choice C is correct because adding 100 to 431 means adding 1 to hundreds digit (4→5), keeping tens and ones same, so hundreds digit changes. This correctly identifies the appropriate place value digit. Choice B represents specific error: said tens digit changes, perhaps confusing +100 with +10. This error typically happens when students don't understand which place value changes with +100 vs +10 or confuse tens and hundreds. To help students: Teach place value explicitly—'Adding 100 means adding 1 hundred. Which place is hundreds? Leftmost digit. So leftmost digit goes up by 1. Tens and ones stay same.' Use place value chart: write 431, show adding 100 changes only hundreds: 4 hundreds → 5 hundreds | 3 tens | 1 one = 531. Practice with hundreds: 'Adding 100 means adding 1 hundred. Which place? Leftmost digit. So leftmost digit goes up by 1. Tens and ones stay same.' Show patterns visually: 331, 431, 531, 631—notice only hundreds digit changes (3→4→5→6), everything else same. Use number line: jump by 100s, notice which digit changes. Connect to skip counting: counting by 100s (300, 400, 500...) uses same skill. Practice mental calculation: 'Don't write. Just change one digit in your head.' Teach exceptions: 398 + 10 requires regrouping (9 tens + 1 ten = 10 tens = 1 hundred 0 tens), 302 - 10 requires borrowing (0 tens, borrow from hundreds). Watch for: changing wrong digit, changing wrong amount, confusing ±10 with ±100, not understanding place value, changing multiple digits.

This question tests 2nd grade understanding of mentally adding and subtracting 10 or 100 to/from three-digit numbers using place value patterns (CCSS 2.NBT.B.8: Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900). To mentally add or subtract 10 or 100, understand which place value digit changes: (1) Adding 10 means adding 1 ten, so tens digit increases by 1 (376 + 10: 7 tens becomes 8 tens = 386; ones and hundreds stay same). (2) Subtracting 10 means subtracting 1 ten, so tens digit decreases by 1 (376 - 10: 7 tens becomes 6 tens = 366). (3) Adding 100 means adding 1 hundred, so hundreds digit increases by 1 (376 + 100: 3 hundreds becomes 4 hundreds = 476; tens and ones stay same). (4) Subtracting 100 means subtracting 1 hundred, so hundreds digit decreases by 1 (528 - 100: 5 hundreds becomes 4 hundreds = 428). Look only at the digit that changes—just add or subtract 1 from that digit, leave other digits alone. This makes calculation easy to do mentally without writing. In this problem, the student must subtract 100 from 781 mentally. To solve mentally, identify operation (-100), identify which digit changes (hundreds for -100), change that digit by 1 (decrease for subtraction), keep other digits same: 781 - 100 → hundreds 7→6, answer 681. Choice B is correct because subtracting 100 from 781 means subtracting 1 from hundreds digit (7→6), keeping tens (8) and ones (1) same, giving 681. This correctly identifies and changes only the appropriate place value digit. Choice C represents a specific error: subtracted 10 instead of 100 (decreased tens digit by 1, 781-10=771 instead of 681). This error typically happens when students confuse -10 with -100, don't understand which place value changes, or subtract from the wrong digit. To help students: Teach place value explicitly—'Subtracting 100 means subtracting 1 hundred. Which place is hundreds? Leftmost digit. So leftmost digit goes down by 1. Tens and ones stay same.' Use place value chart: write 781, show subtracting 100 changes only hundreds: 7 hundreds → 6 hundreds | 8 tens | 1 ones = 681. Practice with tens: 'Subtracting 10 means subtracting 1 ten. Which place? Middle digit. So middle digit goes down by 1. Hundreds and ones stay same.' Show patterns visually: 850, 750, 650, 550—notice only hundreds digit changes (8→7→6→5), everything else same. Use number line: jump backward by 10s or 100s, notice pattern. Connect to skip counting: counting backward by 10s (340, 330, 320...) or by 100s (400, 300, 200...) uses same skill. Practice mental calculation: 'Don't write. Just change one digit in your head.' Teach exceptions: 398 + 10 requires regrouping (9 tens + 1 ten = 10 tens = 1 hundred 0 tens), 302 - 10 requires borrowing (0 tens, borrow from hundreds). Watch for: changing wrong digit, changing wrong amount, confusing -10 with -100, not understanding place value, changing multiple digits.

This question tests 2nd grade understanding of mentally adding and subtracting 10 or 100 to/from three-digit numbers using place value patterns (CCSS 2.NBT.B.8: Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900). To mentally add or subtract 10 or 100, understand which place value digit changes: (1) Adding 10 means adding 1 ten, so tens digit increases by 1 (376 + 10: 7 tens becomes 8 tens = 386; ones and hundreds stay same). (2) Subtracting 10 means subtracting 1 ten, so tens digit decreases by 1 (376 - 10: 7 tens becomes 6 tens = 366). (3) Adding 100 means adding 1 hundred, so hundreds digit increases by 1 (376 + 100: 3 hundreds becomes 4 hundreds = 476; tens and ones stay same). (4) Subtracting 100 means subtracting 1 hundred, so hundreds digit decreases by 1 (528 - 100: 5 hundreds becomes 4 hundreds = 428). Look only at the digit that changes—just add or subtract 1 from that digit, leave other digits alone. This makes calculation easy to do mentally without writing. In this problem, the student must add 100 to 430 mentally. To solve mentally, identify operation (+100), identify which digit changes (hundreds for +100), change that digit by 1 (increase for addition), keep other digits same: 430 + 100 → hundreds 4→5, answer 530. Choice A is correct because adding 100 to 430 means adding 1 to hundreds digit (4→5), keeping tens (3) and ones (0) same, giving 530. This correctly identifies and changes only the appropriate place value digit. Choice B represents a specific error: added 10 instead of 100 (increased tens digit by 1, 430+10=440 instead of 530). This error typically happens when students confuse +10 with +100, don't understand which place value changes, or add to the wrong digit. To help students: Teach place value explicitly—'Adding 100 means adding 1 hundred. Which place? Leftmost digit. So leftmost digit goes up by 1. Tens and ones stay same.' Use place value chart: write 430, show adding 100 changes only hundreds: 4 hundreds → 5 hundreds | 3 tens | 0 ones = 530. Practice with tens: 'Adding 10 means adding 1 ten. Which place? Middle digit. So middle digit goes up by 1. Hundreds and ones stay same.' Show patterns visually: 320, 330, 340, 350—notice only tens digit changes (2→3→4→5), everything else same. Use number line: jump by 10s or 100s, notice pattern. Connect to skip counting: counting by 10s (320, 330, 340...) or by 100s (200, 300, 400...) uses same skill. Practice mental calculation: 'Don't write. Just change one digit in your head.' Teach exceptions: 398 + 10 requires regrouping (9 tens + 1 ten = 10 tens = 1 hundred 0 tens), 302 - 10 requires borrowing (0 tens, borrow from hundreds). Watch for: changing wrong digit, changing wrong amount, confusing +10 with +100, not understanding place value, changing multiple digits.

This question tests 2nd grade understanding of mentally adding and subtracting 10 or 100 to/from three-digit numbers using place value patterns (CCSS 2.NBT.B.8: Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900). To mentally add or subtract 10 or 100, understand which place value digit changes: (1) Adding 10 means adding 1 ten, so tens digit increases by 1 ($376 + 10$: $7$ tens becomes $8$ tens = $386$; ones and hundreds stay same). (2) Subtracting 10 means subtracting 1 ten, so tens digit decreases by 1 ($376 - 10$: $7$ tens becomes $6$ tens = $366$). (3) Adding 100 means adding 1 hundred, so hundreds digit increases by 1 ($376 + 100$: $3$ hundreds becomes $4$ hundreds = $476$; tens and ones stay same). (4) Subtracting 100 means subtracting 1 hundred, so hundreds digit decreases by 1 ($528 - 100$: $5$ hundreds becomes $4$ hundreds = $428$). Describe pattern: Look only at the digit that changes—just add or subtract 1 from that digit, leave other digits alone. This makes calculation easy to do mentally without writing. In this problem, student must subtract 10 from 805 mentally. To solve mentally, identify operation (-10), identify which digit changes (tens for -10), change that digit by 1 (decrease for subtraction), keep other digits same: $805 - 10$ → tens $0 \to 9$ (borrowing from hundreds), but since 0 tens, borrow: hundreds $8 \to 7$, tens $0 \to 9$ (actually $10-1=9$), ones 5 same, answer 795. Choice A is correct because subtracting 10 from 805 means subtracting 1 from tens digit, but with borrowing (0 tens can't subtract 1, so borrow from hundreds: 8 hundreds become 7, 0 tens become 10, then $10-1=9$ tens), keeping ones (5) same, giving 795. This correctly identifies and changes only the appropriate place value digit with borrowing. Choice B represents specific error: subtracted 100 instead of 10, giving 705 instead of 795. This error typically happens when students confuse subtracting 10 with subtracting 100 or don't understand which place value changes with -10 vs -100. To help students: Teach place value explicitly—'Subtracting 10 means subtracting 1 ten. Which place is tens? Middle digit. So middle digit goes down by 1. Hundreds and ones stay same.' Use place value chart: write 805, show subtracting 10 changes only tens, but borrow: 8 hundreds → 7 hundreds | 0 tens → 9 tens | 5 ones = 795. Practice with hundreds: 'Subtracting 100 means subtracting 1 hundred. Which place? Leftmost digit. So leftmost digit goes down by 1. Tens and ones stay same.' Show patterns visually: 825, 815, 805, 795—notice only tens digit changes ($2 \to 1 \to 0 \to 9$), with borrowing when needed. Use number line: jump by 10s backward, notice pattern. Connect to skip counting: counting backward by 10s ($820, 810, 800...$) uses same skill. Practice mental calculation: 'Don't write. Just change one digit in your head.' Teach exceptions: $398 + 10$ requires regrouping ($9$ tens + $1$ ten = $10$ tens = $1$ hundred $0$ tens), $302 - 10$ requires borrowing ($0$ tens, borrow from hundreds). Watch for: changing wrong digit, changing wrong amount, confusing ±10 with ±100, not understanding place value, changing multiple digits.