Question

3/5 of the 30 students are boys how many students are girls

Answer 1

if 3/5 are boys

1- 3/5

5/5 -3/5 = 2/5  are girls

2/5 * 30 = 60/5 = 12

12 are girls

Answer 2

$\frac{3}{5} = \frac{x}{30}$
In this equations x = the number of boys in class. So we are trying to find 3/5 of 30. First cross multiply to simplify the equation.
$5x = 90$
Divide 90 by 5 in order to get the "x" by itself, and you get 18 boys. Then take the number of boys (18) and subtract it from the total number of students (30). The final answer is 12 girls.