There were an equal number of girls in Class U and Class V. ⁵₆ of the pupils in Class U and ¹₅ of the pupils in Class V are boys. If there are 57 more boys in Class U than in Class V, what is the total number of pupils in Class U and Class V?
Fraction of the pupils in Class U that are girls
= 1 - ⁵₆
=  ¹₆

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

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

 1x46x4 Class U = 45 Class V 
 424 Class U = 45 Class V

Number of girls in each class = 4 u

Total number of pupils in Class U = 24 u
Total number of pupils in Class V = 5 u

Number of boys in Class U
= 24 u - 4 u
= 20 u

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

Number of more boys in Class U than Class V
= 20 u - 1 u
= 19 u

19 u = 57

1 u = 57 ÷ 19 = 3

Total number of pupils
= 24 u + 5 u
= 29 u
= 29 x 3
= 87

Answer(s): 87