Start with

Add 2x to both sides:

Subtract 21 from both sides:

Divide both sides by 5. Since it is positive, we don't have to change the inequality sign:

So, the answer is 
I have no idea I am only in Middle School
3+2=5
5x=100%
x=20 (divide)
20*3=60
60% are boys, 40% are girls
Letter B I have done this before