The relationship between fractions and division is one which many children fail to grasp. Put simply, one fifth of 30 is equivalent to 30 divided by 5, or written as a fraction 30 over 5.
It can be a great help to see a fraction as a division calculation. 1/2 can also be thought of as one divided by two.
This page takes a quick look at this and should show whether your child does understand this important relationship.