A doughnut costs £3
A cake costs 50p
This is because from the question you can see that buying 4 more doughnuts costed £12 more
So if 4 = £12, 1 = £3
Now you have the price of a single doughnut you can work out the price of a cake.
3 doughnuts would cost 3 x £3 so £9
Then subtract this from £11 to find the price of 4 cakes.
£11 - £9 = £2
So if 4 cakes = £2, 1 cake = 50p