Question

In a class, 60% of the students are girls. There are 18 girls in the class. How many boys are in the class? How many students are in the class?

Answer 1

Percentage of students that are girls in the class = 60%
Number of girls in the class = 18
Let us assume the total number of students in the class = x
Then
60% * x = 18
(60/100) * x = 18
6x/10 = 18
6x = 18 * 10
6x = 180
x = 180/6
   = 30
So the total number of students in the class is 30.
Number of boys in the class = 30 - 18
                                               = 12
So there are 12 boys in the class. I hope the procedure is simple enough for you to understand.

Answer 2

60% are girls, there are 18 girls in the class. How many boys?

This is quite easy to understand. 

6x/10 = 18
6x/10 *10 = 18*10 
6x = 180
x = 30

Now, thats the total, subtract by the girls. 
30-18 = 12

Answer: 12