percentage=(total amount lost) divided by (initial weight)
Then you just have to multiply by 100 to get into terms of a percentage. Where are you stuck at?
At the start of his diet, Kerrin weighed 15 stone 8 pounds. Three months later he weighed 14 stone 6 pounds. There are 14 pounds in a stone. Calculate the reduction in Kerrin's weight as a percentage of his initial weight, correct to 1 d.p.