So when you have something like 'split up £54 in ratio 4:5', it really just means split £54 into 4 parts and 5 parts. But we can't do that without working out 1 part.
So the way to find 1 part is to add up all the parts (4 + 5) and divide 54 by that number.
So 4 + 5 is 9, and 54 divided by 9 is 6.
So if 1 part is 6, that means 4 parts is 4 times 6, which is 24.
Equally, if 1 part is 6, 5 parts is 5 times 6, which is 30.
Therefore, £54 split up in a ratio of 4:5 is 24:30.