There were an equal number of boys in Class M and Class N. ²₃ of the pupils in Class M and ¹₄ of the pupils in Class N are girls. If there are 40 more girls in Class M than in Class N, what is the total number of pupils in Class M and Class N?
Fraction of the pupils in Class M that are boys
= 1 - ²₃
=  ¹₃

Fraction of the pupils in Class N that are boys
= 1 - ¹₄
= ³₄ 

The number of boys in Class M is equal to the number of boys in Class N. LCM of 1 and 3 is 3. Make the numerators the same. 

 1x33x3 Class M = 34 Class N 
 39 Class M = 34 Class N

Number of boys in each class = 3 u

Total number of pupils in Class M = 9 u
Total number of pupils in Class N = 4 u

Number of girls in Class M
= 9 u - 3 u
= 6 u

Number of girls in Class N
= 4 u - 3 u
= 1 u

Number of more girls in Class M than Class N
= 6 u - 1 u
= 5 u

5 u = 40

1 u = 40 ÷ 5 = 8

Total number of pupils
= 9 u + 4 u
= 13 u
= 13 x 8
= 104

Answer(s): 104