Class 3C and Class 3D have a total of 90 students. ¹₇ of the students in Class 3C and ⁵₁₁ of the students in Class 3D are boys. The number of girls in Class 3C is equal to the number of girls in Class 3D. Find the total number of girls in the 2 classes.
Fraction of Class 3C that are girls
= 1 - ¹₇ 
= ⁶₇

Fraction of Class 3D that are girls
= 1 - ⁵₁₁ 
= ⁶₁₁ 
 
The number of girls in Class 3C is equal to the number of girls in Class 3D.

The numerators are the same.  

⁶₇ Class 3C = ⁶₁₁ Class 3D 

Number of students in Class 3C = 7 u
Number of students in Class 3D = 11 u

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

18 u = 90

1 u = 90 ÷ 18 = 5

Number of girls in Class 3C =6 u
Number of girls in Class 3D =6 u

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

Answer(s): 60