There were an equal number of girls in Class A and Class B. ³₄ of the pupils in Class A and ¹₅ of the pupils in Class B are boys. If there are 66 more boys in Class A than in Class B, what is the total number of pupils in Class A and Class B?
Fraction of the pupils in Class A that are girls
= 1 - ³₄
=  ¹₄

Fraction of the pupils in Class B that are girls
= 1 - ¹₅
= ⁴₅ 

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

 1x44x4 Class A = 45 Class B 
 416 Class A = 45 Class B

Number of girls in each class = 4 u

Total number of pupils in Class A = 16 u
Total number of pupils in Class B = 5 u

Number of boys in Class A
= 16 u - 4 u
= 12 u

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

Number of more boys in Class A than Class B
= 12 u - 1 u
= 11 u

11 u = 66

1 u = 66 ÷ 11 = 6

Total number of pupils
= 16 u + 5 u
= 21 u
= 21 x 6
= 126

Answer(s): 126