There were an equal number of girls in Class C and Class D. ⁴₅ of the pupils in Class C and ¹₅ of the pupils in Class D are boys. If there are 105 more boys in Class C than in Class D, what is the total number of pupils in Class C and Class D?
Fraction of the pupils in Class C that are girls
= 1 - ⁴₅
=  ¹₅

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

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

 1x45x4 Class C = 45 Class D 
 420 Class C = 45 Class D

Number of girls in each class = 4 u

Total number of pupils in Class C = 20 u
Total number of pupils in Class D = 5 u

Number of boys in Class C
= 20 u - 4 u
= 16 u

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

Number of more boys in Class C than Class D
= 16 u - 1 u
= 15 u

15 u = 105

1 u = 105 ÷ 15 = 7

Total number of pupils
= 20 u + 5 u
= 25 u
= 25 x 7
= 175

Answer(s): 175