There is a few ways to do this but I will do it the way that will be shown. You know that you must split the clothing into 4 pieces because 1+3=4
So divide 48 by 4 which is 12. Now multiply 12 by each ration. 
12×1=12
12×3=36 
12+36=48 which means its right 
So the ratio is 12:36