If you look at the numbers you are given, you see that the first purchase has 3 more adult tickets than the second purchase, and its cost is £24 more. This means an adult ticket costs £24/3 = £8.
Two adult tickets will cost 2×£8 = £16, so three child tickets cost ...
£31 -16 = £15
Each child ticket is then £15/3 = £5.
An adult ticket costs £8; a child ticket costs £5.
