math  fractions
posted by Johanna .
Hello, I need help for my 5th grader...Solve the problem by working backward. Write the ans. in a compl. sentence. Mara followed a recipe she found on the net to make her own perfume. She started by adding jojoba oil to a small bottle. Then she added 2 1/3ozs of sandalwood oil, 1 1/4ozs of musk oil, and 1 5/12ozs of frankincense oil. Mara made a total of 8ozs of perfume. How much jojoba oil did she use?

First, let's find the common denominators and add the fractions.
2 1/3 = 2 4/12
1 1/4 = 1 3/12
1 5/12 = 1 5/12
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. . . . 4 12/12 = 5
Subtract 5 from 8 to find the amount of jojoba oil Mara used. 
In your opinion , when would it be advantageous to record a fractional answer in each of following forms? a mixed number, an improper number, a decimal.