There were an equal number of boys in Class H and Class J. ⁴₅ of the students in Class H and ¹₃ of the students in Class J are girls. If there are 28 more girls in Class H than in Class J, what is the total number of students in Class H and Class J?
Fraction of the students in Class H that are boys
= 1 - ⁴₅
=  ¹₅

Fraction of the students in Class J that are boys
= 1 - ¹₃
= ²₃ 

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

 1x25x2 Class H = 23 Class J 
 210 Class H = 23 Class J

Number of boys in each class = 2 u

Total number of students in Class H = 10 u
Total number of students in Class J = 3 u

Number of girls in Class H
= 10 u - 2 u
= 8 u

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

Number of more girls in Class H than Class J
= 8 u - 1 u
= 7 u

7 u = 28

1 u = 28 ÷ 7 = 4

Total number of students
= 10 u + 3 u
= 13 u
= 13 x 4
= 52

Answer(s): 52