Question

If 25 counters are a whole set, how many are there in four fifths of the set?

Answer 1

$$25\cdot\dfrac{4}{5}=20$$

If there are $25:5=5$ countries in one fifth of the set, then there are $4\cdot5=20$ countries in four fifths of the set.

There are $20$ countries in four fifths of the set.