If I'm correct, 100p must be in £1. Since we know that, we can convert it into an equation, like so:
100p = £1
Now, since there are £12, multiply both sides of the equation by 12.
1200p = £12
We're also given 50p to determine what fraction of £12 is from 50p. So place 50p over the total amount of pence, which is 1200.