Class 4R and Class 4S have a total of 40 students. ¹₃ of the students in Class 4R and ³₅ of the students in Class 4S are girls. The number of boys in Class 4R is equal to the number of boys in Class 4S. Find the total number of boys in the 2 classes.
Fraction of Class 4R that are boys
= 1 - ¹₃ 
= ²₃

Fraction of Class 4S that are boys
= 1 - ³₅ 
= ²₅ 
 
The number of boys in Class 4R is equal to the number of boys in Class 4S.

The numerators are the same.  

²₃ Class 4R = ²₅ Class 4S 

Number of students in Class 4R = 3 u
Number of students in Class 4S = 5 u

Total number of students 
= 3 u + 5 u
= 8 u

8 u = 40

1 u = 40 ÷ 8 = 5

Number of boys in Class 4R =2 u
Number of boys in Class 4S =2 u

Total number of boys
= 2 u + 2 u
= 4 u
= 4 x 5
= 20

Answer(s): 20