When you are trying to find a fraction of a number such as your 1/10th of 200, simply divide the number by the denominator. However, when you have a fraction with something other than 1 in the numerator, its a little different. What I personally like to do (there's multiple ways to do this), is I first take the number and divide it by the denominator
Ex. 4/5 of 120
I would divide 120 by 5 do id get 24.
After doing that i'd multiply the number I just got by the numerator.
24 * 4 in this case
so i'd end up with 96