You should have learned it at school although i'm not really sure wether this was the right translation or not.
if x/y equals a/b, you can calculate x as follows:
x=(a*y)/b with a,b and y given.
another example: 5 bananas need 40% of your fruit jars, how many bananas can be in your jars at all?
5/40 = x/100, x = (5*100)/40 = 12,5 bananas.
easy as can be.