Students need to recognise how the concepts of twice and half are related. If one amount is twice another, the second is half the first and vice-versa.
So, in this problem, the number of 5 pence pieces must equal the number of 20 pence pieces. In addition, they must figure out that if there are half as many 20 pence pieces as 10 pence pieces, the number of 10 pence pieces has to be even.
But since the number of 10 pence pieces is twice the number of 5 pence pieces, it will automatically be even. The task also provides students practice in adding money amounts.