Maths Learning

I have learnt how to work out fraction of a set problems/Equations.

For example -

C. Rebecca ate two pieces of the apple pie, what fraction of the pie did she eat?

 - 2/5

Rebecca has $25 and she gave ⅖ of her money to her sister.

How much does she got left?

5 x 5=$25

1/5 of $25 = $5

2/5 of $25 = 10

5 x 2=$10

25-10=15

Answer - 15