There were an equal number of girls in Class N and Class P. ²₃ of the pupils in Class N and ¹₄ of the pupils in Class P are boys. If there are 15 more boys in Class N than in Class P, what is the total number of pupils in Class N and Class P?
Fraction of the pupils in Class N that are girls
= 1 - ²₃
=  ¹₃

Fraction of the pupils in Class P that are girls
= 1 - ¹₄
= ³₄ 

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

 1x33x3 Class N = 34 Class P 
 39 Class N = 34 Class P

Number of girls in each class = 3 u

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

Number of boys in Class N
= 9 u - 3 u
= 6 u

Number of boys in Class P
= 4 u - 3 u
= 1 u

Number of more boys in Class N than Class P
= 6 u - 1 u
= 5 u

5 u = 15

1 u = 15 ÷ 5 = 3

Total number of pupils
= 9 u + 4 u
= 13 u
= 13 x 3
= 39

Answer(s): 39