There were an equal number of girls in Class T and Class U. ⁴₅ of the students in Class T and ¹₅ of the students in Class U are boys. If there are 75 more boys in Class T than in Class U, what is the total number of students in Class T and Class U?
Fraction of the students in Class T that are girls
= 1 - ⁴₅
=  ¹₅

Fraction of the students in Class U that are girls
= 1 - ¹₅
= ⁴₅ 

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

 1x45x4 Class T = 45 Class U 
 420 Class T = 45 Class U

Number of girls in each class = 4 u

Total number of students in Class T = 20 u
Total number of students in Class U = 5 u

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

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

Number of more boys in Class T than Class U
= 16 u - 1 u
= 15 u

15 u = 75

1 u = 75 ÷ 15 = 5

Total number of students
= 20 u + 5 u
= 25 u
= 25 x 5
= 125

Answer(s): 125