There were an equal number of girls in Class R and Class S. ²₃ of the students in Class R and ¹₅ of the students in Class S are boys. If there are 70 more boys in Class R than in Class S, what is the total number of students in Class R and Class S?
Fraction of the students in Class R that are girls
= 1 - ²₃
=  ¹₃

Fraction of the students in Class S that are girls
= 1 - ¹₅
= ⁴₅ 

The number of girls in Class R is equal to the number of girls in Class S. LCM of 1 and 4 is 4. Make the numerators the same. 

 1x43x4 Class R = 45 Class S 
 412 Class R = 45 Class S

Number of girls in each class = 4 u

Total number of students in Class R = 12 u
Total number of students in Class S = 5 u

Number of boys in Class R
= 12 u - 4 u
= 8 u

Number of boys in Class S
= 5 u - 4 u
= 1 u

Number of more boys in Class R than Class S
= 8 u - 1 u
= 7 u

7 u = 70

1 u = 70 ÷ 7 = 10

Total number of students
= 12 u + 5 u
= 17 u
= 17 x 10
= 170

Answer(s): 170