There were an equal number of boys in Class B and Class C. ⁴₅ of the students in Class B and ¹₃ of the students in Class C are girls. If there are 70 more girls in Class B than in Class C, what is the total number of students in Class B and Class C?
Fraction of the students in Class B that are boys
= 1 - ⁴₅
=  ¹₅

Fraction of the students in Class C that are boys
= 1 - ¹₃
= ²₃ 

The number of boys in Class B is equal to the number of boys in Class C. LCM of 1 and 2 is 2. Make the numerators the same. 

 1x25x2 Class B = 23 Class C 
 210 Class B = 23 Class C

Number of boys in each class = 2 u

Total number of students in Class B = 10 u
Total number of students in Class C = 3 u

Number of girls in Class B
= 10 u - 2 u
= 8 u

Number of girls in Class C
= 3 u - 2 u
= 1 u

Number of more girls in Class B than Class C
= 8 u - 1 u
= 7 u

7 u = 70

1 u = 70 ÷ 7 = 10

Total number of students
= 10 u + 3 u
= 13 u
= 13 x 10
= 130

Answer(s): 130