Add and subtract fractions with unlike denominators: word problems

Learn with an example

Last week Sharon and her roommates ate 1/2 of a carton of yoghurt, and this week they ate 5/6 of a carton.

How much more yoghurt did they eat this week compared to last week?

Find how many more cartons they ate this week by subtracting 1/2 from 5/6.

You can use 6 as the common denominator.
Multiply the numerator and the denominator of 1/2 by 3 to find an equivalent fraction.

1 × 3 = 3
2 × 3 6

Now subtract the fractions. Subtract the numerators and keep the denominators the same.

5/6 -3/6 = 2/6

Sharon and her roommates ate 2/6 of a carton more yoghurt this week.

At Jill’s Boutique, 1/12 of the trousers are denim. Another 1/2 is khaki.

What fraction of the trousers are either denim or khaki?

Find the fraction of the trousers that are either denim or khaki by adding 1/12 and 1/2.

You can use 12 as the common denominator.

Multiply the numerator and the denominator of 1/2 by 6 to find an equivalent fraction.

1×6 / 2×6 = 6/12

Now add the fractions. Add the numerators and keep the denominators the same.

1/12+6/12 = 7/12

The fraction of the trousers that are either denim or khaki is 7/12.

While making desserts for a bake sale, Zane used 2/5 of a scoop of brown sugar as well as 3/10 of a scoop of white sugar.

How much more brown sugar did Zane use?

Find how much more brown sugar was used by subtracting 3/10 from 2/5.

You can use 10 as the common denominator.
Multiply the numerator and the denominator of 2/5 by 2 to find an equivalent fraction.

2×2 / 5×2 = 4/10

Now subtract the fractions. Subtract the numerators and keep the denominators the same.

4/10 – 3/10 = 1/10

Zane used 1/10 of a scoop more brown sugar.

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