There were an equal number of boys in Class Y and Class Z. ³₄ of the pupils in Class Y and ¹₃ of the pupils in Class Z are girls. If there are 15 more girls in Class Y than in Class Z, what is the total number of pupils in Class Y and Class Z?
Fraction of the pupils in Class Y that are boys
= 1 - ³₄
=  ¹₄

Fraction of the pupils in Class Z that are boys
= 1 - ¹₃
= ²₃ 

The number of boys in Class Y is equal to the number of boys in Class Z. LCM of 1 and 2 is 2. Make the numerators the same. 

 1x24x2 Class Y = 23 Class Z 
 28 Class Y = 23 Class Z

Number of boys in each class = 2 u

Total number of pupils in Class Y = 8 u
Total number of pupils in Class Z = 3 u

Number of girls in Class Y
= 8 u - 2 u
= 6 u

Number of girls in Class Z
= 3 u - 2 u
= 1 u

Number of more girls in Class Y than Class Z
= 6 u - 1 u
= 5 u

5 u = 15

1 u = 15 ÷ 5 = 3

Total number of pupils
= 8 u + 3 u
= 11 u
= 11 x 3
= 33

Answer(s): 33