There were an equal number of boys in Class U and Class V. ⁴₅ of the pupils in Class U and ¹₅ of the pupils in Class V are girls. If there are 135 more girls 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 boys
= 1 - ⁴₅
=  ¹₅

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

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

 1x45x4 Class U = 45 Class V 
 420 Class U = 45 Class V

Number of boys in each class = 4 u

Total number of pupils in Class U = 20 u
Total number of pupils in Class V = 5 u

Number of girls in Class U
= 20 u - 4 u
= 16 u

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

Number of more girls in Class U than Class V
= 16 u - 1 u
= 15 u

15 u = 135

1 u = 135 ÷ 15 = 9

Total number of pupils
= 20 u + 5 u
= 25 u
= 25 x 9
= 225

Answer(s): 225