Question

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?

Answer 1

X- amount of girls
y-amount of boys

x+y=51
y+12=2*x

From first equation:
x=51-y

substitude to second:
y+12=2*(51-y)

y+12=102-2y
3y=102-12

3y=90 /:3
y=30

x=51-30=21 

There are 21 girls.

Answer 2

$b-boys\\g-girls\\\\ \left\{\begin{array}{ccc}b+g=51\\2g=b+12\end{array}\right\\\\+\left\{\begin{array}{ccc}b+g=51\\2g-b=12\end{array}\right\\----------\\.\ \ \ \ \ \ 3g=63\ \ \ \ /:3\\.\ \ \ \ \ \ g=21\\\\Answer:21\ girls.$