Question

the student to teacher ratio is 15:3. if there are 90 teachers at the school, how many students are there

Answer 1

$15x+3x=90\\ 18x=90\\ x=5\\\\ 15x=15\cdot5=\boxed{75}$

Answer 2

90/3 = 30
30*15=450 Students

To check: 15:3  (divide by three)
                   5:1
So should be five times as many students.
90*5=450 :) Hope this helps :)