Number of boys: number of girls=11: 9there are 124 boys more than girls. what is the total number of student at the school?

boys (b): g + 124

girls (g): g

Total (T): b + g

I am going to set up a proportion based on boys to total students.

⇒  

⇒ 11(124 + 2g) = 20(124 + g)

⇒ 1364 + 22g = 2480 + 20g

⇒ 1364  + 2g = 2480

⇒              2g = 1116

⇒                g = 558

boys (b): g + 124   = (558) + 124   = 682

Total (T): b + g   = 682 + 558   = 1240

1240 total students

$12. that is my answer.