Jonathan’s class has 30 boys. of the students in his class, 60% are girls. how many girls are in jonathan’s class?
(a)75
(b)60
(c)45
(d)20

Let x = the number of total students in the class

Convert the percent into a decimal by moving the percent sign two places to the left and change the percent sign into a decimal point.

60% = 0.6

Then 0.6x = the number of girls in the class
Then 0.6x + 30 = the total number of students in the class

0.6x + 30 = x

 Solve for x.

0.6x + 30 = x
0.6x = x - 30 <-- Subtract both sides by 30
-0.4x = -30 <-- Subtract both sides by x
x = 75

Now we have the total number of students in the class. 

60 percent of the students are female.

0.6 * 75 = 45

So, C 45 is the answer.