Addition is the first step for two reasons. 1) the Order of Operations requires you do operations in parentheses first 2) the division cannot be performed until you know what you're dividing
Since the ratio of Boys to Girls is 4/3, set this equal to X number of boys/36. Cross multiply so 36*4=144, and 3*x= 3x. So now you have 144=3x, divide by 3 to get x=48 boys.