Add and Subtract Unlike Fractions
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5th Grade Math › Add and Subtract Unlike Fractions
On a hike, Noah walked $\frac{3}{5}$ mile and then $\frac{1}{2}$ mile. Both distances are parts of the same 1-mile whole. He rewrote them as $\frac{3}{5}=\frac{6}{10}$ and $\frac{1}{2}=\frac{5}{10}$. Which statement about the size of the result is correct?
The total is exactly 1 mile because $3+1=4$ and $5+2=7$.
The total is less than 1 mile because the denominators are different.
The total is $\frac{4}{7}$ mile because you add the numerators and denominators.
The total is greater than 1 mile because $\frac{6}{10}+\frac{5}{10}=\frac{11}{10}$.
Explanation
When adding or subtracting unlike fractions, we first need to find equivalent fractions with the same denominator. To do this, identify a common denominator, such as 10 for denominators 5 and 2, which is the least common multiple. Then, rewrite each fraction: multiply the numerator and denominator of $\frac{3}{5}$ by 2 to get $\frac{6}{10}$, and of $\frac{1}{2}$ by 5 to get $\frac{5}{10}$. Once the fractions have the same denominator, add the numerators $6 + 5 = 11$ while keeping the denominator 10, resulting in $\frac{11}{10}$ mile, which is greater than 1. A common misconception is that different denominators mean the sum is less than 1, but equivalents show otherwise. Using equivalent fractions ensures accurate size comparisons of the result. This method generalizes to estimating totals in activities like hiking, providing reliable insights into quantities.
A class has a same-sized 1-hour study block. Students spend $\frac{3}{5}$ of the hour reading and $\frac{1}{2}$ of the hour writing. The fractions must refer to the same whole (1 hour). A student rewrites them as $\frac{3}{5}=\frac{6}{10}$ and $\frac{1}{2}=\frac{5}{10}$. What fraction of the hour is spent reading and writing altogether?
$\frac{4}{5}$ of the hour
$\frac{4}{10}$ of the hour
$\frac{11}{10}$ of the hour
$\frac{4}{7}$ of the hour
Explanation
When adding or subtracting unlike fractions, which have different denominators, we need to convert them to equivalent fractions with the same denominator. To do this, find a common denominator, such as the least common multiple of 5 and 2, which is 10. Rewrite each fraction by multiplying the numerator and denominator by the same number: 3/5 becomes (3×2)/(5×2) = 6/10, and 1/2 becomes (1×5)/(2×5) = 5/10. Now, add the numerators while keeping the common denominator: 6/10 + 5/10 = 11/10 of the hour spent. A common misconception is adding without a common denominator, leading to wrong sums like 4/7. Using equivalent fractions allows combination by equalizing units. This equivalence makes fraction addition and subtraction possible across different denominators.
A garden bed is the same whole garden bed for both measurements. One section is $\frac{7}{8}$ full of soil, and another section adds $\frac{2}{5}$ of the bed more. The fractions must refer to the same whole (one garden bed). A student rewrites them as $\frac{7}{8}=\frac{35}{40}$ and $\frac{2}{5}=\frac{16}{40}$. What is the total fraction of the garden bed filled?
$\frac{9}{13}$ of the bed
$\frac{9}{40}$ of the bed
$\frac{37}{13}$ of the bed
$\frac{51}{40}$ of the bed
Explanation
When adding or subtracting unlike fractions, which have different denominators, we need to convert them to equivalent fractions with the same denominator. To do this, find a common denominator, such as the least common multiple of 8 and 5, which is 40. Rewrite each fraction by multiplying the numerator and denominator by the same number: 7/8 becomes (7×5)/(8×5) = 35/40, and 2/5 becomes (2×8)/(5×8) = 16/40. Now, add the numerators while keeping the common denominator: 35/40 + 16/40 = 51/40 of the bed filled. A common misconception is that adding fractions means adding numerators while keeping one denominator, but this ignores equivalence. Using equivalent fractions allows precise combination by equalizing part sizes. This principle generalizes to enable addition and subtraction across any unlike fractions.
A recipe uses flour measured with the same 1-cup measuring cup. You already added $1\frac{1}{4}$ cups of flour, but the recipe needs $2\frac{1}{3}$ cups total. The fractions must refer to the same whole (1 cup). Using equivalent fractions, $\frac{1}{4}=\frac{3}{12}$ and $\frac{1}{3}=\frac{4}{12}$. How much more flour is needed?
$1\frac{1}{12}$ cups
$\frac{1}{12}$ cup
$\frac{3}{7}$ cup
$1\frac{2}{7}$ cups
Explanation
When adding or subtracting unlike fractions, which have different denominators, we need to convert them to equivalent fractions with the same denominator. To do this, find a common denominator, such as the least common multiple of 3 and 4, which is 12. Rewrite each fraction by multiplying the numerator and denominator by the same number: $2 \frac{1}{3}$ is $\frac{7}{3}$, which becomes $(7\times4)/(3\times4) = \frac{28}{12}$, and $1 \frac{1}{4}$ is $\frac{5}{4}$, which becomes $(5\times3)/(4\times3) = \frac{15}{12}$. Now, subtract the numerators while keeping the common denominator: $\frac{28}{12} - \frac{15}{12} = \frac{13}{12} = 1 \frac{1}{12}$ cups more needed. A common misconception is subtracting mixed numbers by only handling wholes or fractions separately without equivalence. Using equivalent fractions makes subtraction possible by standardizing denominators. This equivalence allows for accurate operations on fractions in various contexts.
A pan of cornbread is cut into equal pieces, and all fractions refer to the same whole pan. Liam ate $\tfrac{3}{10}$ of the pan. Later he ate $\tfrac{1}{2}$ of the pan. He rewrites $\tfrac{1}{2}$ as $\tfrac{5}{10}$ to use equivalent fractions. What fraction of the pan did Liam eat in all?
$\tfrac{8}{10}$ of the pan
$\tfrac{4}{20}$ of the pan
$\tfrac{4}{5}$ of the pan
$\tfrac{4}{12}$ of the pan
Explanation
To add or subtract unlike fractions, which have different denominators, we must first convert them to equivalent fractions with the same denominator to ensure they refer to parts of the same-sized whole. We find a common denominator by identifying a common multiple of the two denominators, preferably the least common multiple, such as 10 for 10 and 2 in this cornbread pan addition. To rewrite the fractions, multiply both the numerator and denominator of each by the same number; for example, 1/2 becomes 5/10. Once they have the same denominator, add the numerators to get 8/10 of the pan total. A common misconception is keeping different denominators and adding numerators directly. Using equivalent fractions allows us to combine or compare parts accurately by making them comparable. This method ensures that operations on fractions are meaningful and applicable in dividing items like pans of food.
Two pitchers each hold the same 1-liter whole (fractions must refer to the same-sized whole). One pitcher has $\frac{7}{8}$ liter of juice. Another has $\frac{1}{3}$ liter of juice. A student writes equivalent fractions: $\frac{7}{8}=\frac{21}{24}$ and $\frac{1}{3}=\frac{8}{24}$. What is the difference $\frac{7}{8}-\frac{1}{3}$ liters?
$\frac{6}{11}$ liter
$\frac{20}{11}$ liter
$\frac{6}{5}$ liter
$\frac{13}{24}$ liter
Explanation
When subtracting unlike fractions such as 7/8 and 1/3, we need to find equivalent fractions with the same denominator to compare precisely. To find a common denominator, use the least common multiple of 8 and 3, which is 24. Rewrite 7/8 as 21/24 by multiplying numerator and denominator by 3, and 1/3 as 8/24 by multiplying by 8. Subtract the numerators: 21 - 8 = 13, over 24, giving 13/24 liter difference. A common misconception is using the wrong common denominator, but LCM ensures efficiency. Equivalents make fractions compatible, enabling subtraction or addition. This principle generalizes to all fraction operations, ensuring correctness.
A recipe uses the same-sized whole cup as the unit. Maya pours $\tfrac{2}{3}$ cup of milk and then adds $\tfrac{1}{4}$ cup more. Because the fractions must refer to the same-sized whole cup, she rewrites them as equivalent fractions with a common denominator: $\tfrac{2}{3}=\tfrac{8}{12}$ and $\tfrac{1}{4}=\tfrac{3}{12}$. What is the total amount of milk she poured?
$\tfrac{9}{12}$ cup
$\tfrac{3}{7}$ cup
$\tfrac{3}{12}$ cup
$\tfrac{11}{12}$ cup
Explanation
To add or subtract unlike fractions, which have different denominators, we must first convert them to equivalent fractions with the same denominator to ensure they refer to parts of the same-sized whole. We find a common denominator by identifying a common multiple of the two denominators, preferably the least common multiple, such as 12 for 3 and 4 in this milk-pouring scenario. To rewrite the fractions, multiply both the numerator and denominator of each by the same number; for example, multiply 2/3 by 4/4 to get 8/12, and 1/4 by 3/3 to get 3/12. Once they have the same denominator, add the numerators while keeping the denominator the same, resulting in 11/12 cup of milk total. A common misconception is that you can simply add the numerators and denominators separately, but this doesn't account for the different part sizes. Using equivalent fractions allows us to combine or compare parts accurately by making them comparable. This method ensures that operations on fractions are meaningful and applicable in real-world measurements like recipes.
A science class pours water into a container, using the same 1-liter measuring cup each time. They pour $\frac{5}{6}$ liter, then pour $\frac{1}{4}$ liter more. The fractions must refer to the same whole (1 liter). One student rewrites them as equivalent fractions: $\frac{5}{6}=\frac{10}{12}$ and $\frac{1}{4}=\frac{3}{12}$. What is the total amount of water poured?
$\frac{11}{24}$ liter
$\frac{6}{12}$ liter
$\frac{13}{12}$ liter
$\frac{6}{10}$ liter
Explanation
When adding or subtracting unlike fractions, which have different denominators, we need to convert them to equivalent fractions with the same denominator. To do this, find a common denominator, such as the least common multiple of 6 and 4, which is 12. Rewrite each fraction by multiplying the numerator and denominator by the same number: 5/6 becomes (5×2)/(6×2) = 10/12, and 1/4 becomes (1×3)/(4×3) = 3/12. Now, add the numerators while keeping the common denominator: 10/12 + 3/12 = 13/12 liter, the total water poured. A common misconception is that fractions with larger denominators are always smaller, but equivalence shows value depends on both numerator and denominator. Using equivalent fractions makes it possible to operate on them by standardizing the part sizes. This method generalizes to all fraction additions and subtractions, allowing accurate comparisons and calculations.
Liam filled a water bottle that holds 1 whole liter. He drank $\frac{7}{8}$ liter and then drank $\frac{1}{3}$ liter more from another identical full bottle. The fractions refer to the same-sized whole liter. He rewrote $\frac{7}{8}=\frac{21}{24}$ and $\frac{1}{3}=\frac{8}{24}$. What is the total amount he drank?
$\frac{8}{11}$ liter
$\frac{8}{24}$ liter
$\frac{22}{24}$ liter
$\frac{29}{24}$ liter
Explanation
When adding or subtracting unlike fractions, we first need to find equivalent fractions with the same denominator. To do this, identify a common denominator, such as 24 for denominators 8 and 3, which is the least common multiple. Then, rewrite each fraction: multiply the numerator and denominator of $\frac{7}{8}$ by 3 to get $\frac{21}{24}$, and of $\frac{1}{3}$ by 8 to get $\frac{8}{24}$. Once the fractions have the same denominator, add the numerators $21 + 8 = 29$ while keeping the denominator 24, resulting in $\frac{29}{24}$ liter drunk. A common misconception is that adding improper fractions directly without equivalents gives a valid result, but this ignores the different part sizes. Using equivalent fractions ensures we combine equal portions of the liter accurately. This method generalizes to adding any unlike fractions, supporting calculations in everyday situations like totaling liquid consumption.
On a number line from 0 to 1 (the same whole), a student wants to add $\frac{3}{8}$ and $\frac{1}{2}$. The student writes equivalent fractions: $\frac{1}{2}=\frac{4}{8}$. Which statement correctly explains how the fractions were combined?
Make equivalent fractions with a common denominator of 8, then add: $\frac{3}{8}+\frac{4}{8}=\frac{7}{8}$.
Add the numerators and denominators: $\frac{3+1}{8+2}=\frac{4}{10}$.
Add only the numerators and keep 8: $\frac{3+1}{8}=\frac{4}{8}$.
Add the denominators: $8+2=10$, so the sum is $\frac{4}{10}$.
Explanation
When adding unlike fractions such as 3/8 and 1/2, we need to find equivalent fractions with the same denominator to combine them effectively. To find a common denominator, note that 8 is a multiple of 2, so use 8. Rewrite 1/2 as 4/8 by multiplying numerator and denominator by 4, while 3/8 stays the same. Add the numerators over the common denominator: 3 + 4 = 7, giving 7/8. A common misconception is adding numerators and denominators separately, like (3+1)/(8+2) = 4/10, but this distorts the values. Equivalent fractions align the parts, making addition possible. This method generalizes to subtracting unlike fractions too, ensuring accurate results.