Class 1C and Class 1D have a total of 90 students. ¹₇ of the students in Class 1C and ⁵₁₁ of the students in Class 1D are girls. The number of boys in Class 1C is equal to the number of boys in Class 1D. Find the total number of boys in the 2 classes.
Fraction of Class 1C that are boys
= 1 - ¹₇ 
= ⁶₇

Fraction of Class 1D that are boys
= 1 - ⁵₁₁ 
= ⁶₁₁ 
 
The number of boys in Class 1C is equal to the number of boys in Class 1D.

The numerators are the same.  

⁶₇ Class 1C = ⁶₁₁ Class 1D 

Number of students in Class 1C = 7 u
Number of students in Class 1D = 11 u

Total number of students 
= 7 u + 11 u
= 18 u

18 u = 90

1 u = 90 ÷ 18 = 5

Number of boys in Class 1C =6 u
Number of boys in Class 1D =6 u

Total number of boys
= 6 u + 6 u
= 12 u
= 12 x 5
= 60

Answer(s): 60