The number of boys in ninth grade is 12 less than twice the number of girls There are 51 students in ninth grade How many are girls?

Let's lay out the known information in equations,

where B is the number of boys and G is the number of girls.

Then we express the number B in terms of G, which is twice

as much less 12, or (2G-12)

B + G = 51 , and

B = 2G - 12

Now, replace the B in the first equation with its value from the second equation

(2G - 12) + G = 51

Remove the brackets

2G - 12 + G = 51

Add 12 to both sides

2G + G = 51 + 12

3G = 63

Divide both sides by 3

G = 21

Now, to check if we're right,

Replace G in the first equation

B + 21 = 51

B = 30

If that's right then

30 = 2(21) - 12

30 = 42 - 12

And it is, so we know that there are 21 girls and 30 boys in ninth grade.