Add and Subtract Unlike Fractions

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5th Grade Math › Add and Subtract Unlike Fractions

Questions 1 - 10
1

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?

$\frac{20}{11}$ liter

$\frac{6}{11}$ liter

$\frac{13}{24}$ liter

$\frac{6}{5}$ 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.

2

Two students are measuring the same 1-yard strip of fabric (fractions must refer to the same-sized whole). One student used $\frac{5}{6}$ yard. The other student used $\frac{1}{4}$ yard. They find equivalent fractions: $\frac{5}{6}=\frac{10}{12}$ and $\frac{1}{4}=\frac{3}{12}$. What is the difference $\frac{5}{6}-\frac{1}{4}$?

$\frac{4}{10}$ yard

$\frac{6}{10}$ yard

$\frac{7}{12}$ yard

$\frac{9}{24}$ yard

Explanation

When subtracting unlike fractions such as 5/6 and 1/4, we need to find equivalent fractions with the same denominator to compare them correctly. To find a common denominator, identify the least common multiple of 6 and 4, which is 12. Rewrite 5/6 as 10/12 by multiplying numerator and denominator by 2, and 1/4 as 3/12 by multiplying by 3. Subtract the numerators over the common denominator: 10 - 3 = 7, resulting in 7/12 yard difference. A common misconception is subtracting denominators too, but denominators define part sizes and stay the same when common. Using equivalents ensures fractions refer to identical part sizes, enabling subtraction. This equivalence enables precise operations on unlike fractions in any addition or subtraction scenario.

3

A water bottle holds 1 whole liter (the same-sized whole). A student drank $1\frac{1}{2}$ liters over the day from refills, then drank $\frac{2}{3}$ liter more. These amounts refer to the same unit (liters). The student rewrites $\frac{1}{2}=\frac{3}{6}$ and $\frac{2}{3}=\frac{4}{6}$. What is $1\frac{1}{2}+\frac{2}{3}$ liters?

$\frac{3}{5}$ liter

$1\frac{3}{5}$ liters

$2\frac{1}{6}$ liters

$1\frac{3}{6}$ liters

Explanation

When adding unlike fractions in mixed numbers like 1 1/2 and 2/3, we need to find equivalents for the fractional parts with the same denominator. To find a common denominator, use the least common multiple of 2 and 3, which is 6. Rewrite 1/2 as 3/6 by multiplying numerator and denominator by 3, and 2/3 as 4/6 by multiplying by 2. Add the fractions to the whole: 1 + 3/6 + 4/6 = 1 + 7/6 = 2 1/6 liters. A common misconception is forgetting to handle the whole number separately, but mixed numbers require combining like parts. Equivalent fractions make both parts comparable, facilitating addition or subtraction. This approach works broadly for unlike fractions, allowing seamless operations across various contexts.

4

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?

Add only the numerators and keep 8: $\frac{3+1}{8}=\frac{4}{8}$.

Add the numerators and denominators: $\frac{3+1}{8+2}=\frac{4}{10}$.

Make equivalent fractions with a common denominator of 8, then add: $\frac{3}{8}+\frac{4}{8}=\frac{7}{8}$.

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.

5

A recipe uses the same 1-cup measuring cup as the whole. You have $2\frac{1}{4}$ cups of flour and you add $\frac{2}{3}$ cup more. You rewrite $\frac{1}{4}=\frac{3}{12}$ and $\frac{2}{3}=\frac{8}{12}$. What is $2\frac{1}{4}+\frac{2}{3}$ cups?

$2\frac{11}{12}$ cups

$2\frac{3}{7}$ cups

$2\frac{3}{12}$ cups

$\frac{3}{7}$ cup

Explanation

When adding unlike fractions in mixed numbers like 2 1/4 and 2/3, we need to find equivalents for the fractional parts with the same denominator. To find a common denominator, use the least common multiple of 4 and 3, which is 12. Rewrite 1/4 as 3/12 by multiplying numerator and denominator by 3, and 2/3 as 8/12 by multiplying by 4. Add to the whole: 2 + 3/12 + 8/12 = 2 + 11/12 = 2 11/12 cups. A common misconception is adding wholes and fractions without converting, but unlike denominators prevent direct combination. Equivalents make parts uniform, allowing proper addition or subtraction. This principle applies universally to fraction operations, promoting consistency.

6

A music playlist is 1 hour long (the same-sized whole). A student listened to $\frac{2}{3}$ of the hour and then listened to $\frac{1}{6}$ of the hour more. The student rewrote $\frac{2}{3}=\frac{4}{6}$. What is the total fraction of the hour the student listened to?

$\frac{3}{18}$ of the hour

$\frac{5}{6}$ of the hour

$\frac{3}{9}$ of the hour

$\frac{3}{12}$ of the hour

Explanation

When adding unlike fractions such as 2/3 and 1/6, we need to find equivalent fractions with the same denominator for accurate combination. To find a common denominator, note that 6 is a multiple of 3, so use 6. Rewrite 2/3 as 4/6 by multiplying numerator and denominator by 2, while 1/6 stays the same. Add the numerators: 4 + 1 = 5, over 6, giving 5/6 of the hour. A common misconception is adding without converting, but different denominators mean unequal parts. Equivalent fractions standardize the parts, allowing addition or subtraction. This technique applies to any unlike fractions, facilitating reliable operations.

7

Tara practiced piano for $1\frac{1}{3}$ hours on Monday and $\frac{3}{4}$ hour on Tuesday. These fractions must refer to the same whole (1 hour). She rewrites $\frac{1}{3}$ as $\frac{4}{12}$ and $\frac{3}{4}$ as $\frac{9}{12}$. What is the total time she practiced?

$2\frac{1}{12}$ hours

$1\frac{4}{7}$ hours

$1\frac{4}{12}$ hours

$2\frac{13}{12}$ hours

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: 1 1/3 is 4/3, which becomes (4×4)/(3×4) = 16/12, and 3/4 becomes (3×3)/(4×3) = 9/12. Now, add the numerators while keeping the common denominator: 16/12 + 9/12 = 25/12 = 2 1/12 hours, the total practice time. A common misconception is to add only the fractional parts without considering the whole numbers in mixed fractions. Using equivalent fractions ensures we can combine them by aligning the denominators. This equivalence facilitates all fraction operations, making it possible to handle diverse denominators effectively.

8

Two students ate parts of the same-sized pizza. One ate $1\frac{1}{2}$ pizzas and the other ate $\frac{3}{4}$ of a pizza. The fractions must refer to the same whole (one pizza). Using equivalent fractions, $\frac{1}{2}=\frac{2}{4}$. What is the total amount of pizza eaten?

$1\frac{2}{6}$ pizzas

$1\frac{4}{8}$ pizzas

$1\frac{4}{6}$ pizzas

$2\frac{1}{4}$ pizzas

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 2 and 4, which is 4. Rewrite each fraction by multiplying the numerator and denominator by the same number: 1 1/2 is 3/2, which becomes (3×2)/(2×2) = 6/4, and 3/4 stays 3/4. Now, add the numerators while keeping the common denominator: 6/4 + 3/4 = 9/4 = 2 1/4 pizzas, the total eaten. A common misconception is to add mixed numbers without converting the fractions properly, leading to errors in the whole parts. Using equivalent fractions allows us to combine values precisely by making the denominators match. This equivalence enables fraction operations in general, ensuring consistent units across different expressions.

9

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

$\frac{51}{40}$ of the bed

$\frac{37}{13}$ of the bed

$\frac{9}{40}$ 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.

10

Two paint cans hold the same-sized 1-gallon can as the whole. One can has $\frac{2}{3}$ gallon of paint left, and the other has $\frac{3}{8}$ gallon left. The fractions must refer to the same whole (1 gallon). A student says, “I can add them by doing $\frac{2+3}{3+8}=\frac{5}{11}$.” Which claim about adding $\frac{2}{3}$ and $\frac{3}{8}$ is incorrect?

You can add the numerators and add the denominators to get $\frac{5}{11}$.

You can rewrite $\frac{2}{3}$ as $\frac{16}{24}$ and $\frac{3}{8}$ as $\frac{9}{24}$ to add.

You should rewrite both fractions using a common denominator before adding.

The sum should be a little more than 1 gallon because $\frac{2}{3}$ is more than $\frac{1}{2}$ and $\frac{3}{8}$ is less than $\frac{1}{2}$.

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 8, which is 24. Rewrite each fraction by multiplying the numerator and denominator by the same number: 2/3 becomes (2×8)/(3×8) = 16/24, and 3/8 becomes (3×3)/(8×3) = 9/24. Now, add the numerators while keeping the common denominator: 16/24 + 9/24 = 25/24 gallon total, showing claim A is incorrect as it adds numerators and denominators wrongly. A common misconception is exactly that—adding numerators and denominators separately, which doesn't preserve value. Using equivalent fractions makes operations possible by aligning denominators. This equivalence allows for correct addition and subtraction of any unlike fractions.

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