3.12
First, write an equation to express what you know.
g = number of girls
b = number of boys
b * 2.52 + g * x = 2.88(b+g)
Solve for x, first substitute known values for g and b
12 * 2.52 + 18 * x = 2.88(12+18)
Add and multiply what you already know
30.24 + 18x = 86.4
Subtract 30.24 from both sides
18x = 56.16
Divide both sides by 18
x = 3.12
Therefore the GPA of the girls is 3.12
