Firstly we know that £1 equals 100p.
Then we could write 50p as £0.5 (As 50/100= 5/10= 1/2= 0.5), which is equivalent.
Now we do the fraction, £0.5 over £4, which is 0.5/4, enlarge both of the numerator and denominator 10 times simly gives us 5/40= 1/8(= 0.125 if needed in decimal form).