Theory
The word of usually means multiply in maths. For Foundation GCSE, it is often easier to do this in two steps.
For 1 / 5 of an amount, divide the amount by 5.
For 3 / 5 of an amount, first find 1 / 5, then multiply by 3.
The denominator tells you how many equal parts the amount is split into.
The numerator tells you how many of those parts you need.
If the numerator is 1, you only need to divide by the denominator.