You buy 12 tickets and the total is $120. a student ticket is $9 and an adult ticket is $12. how many student tickets did you buy

4 adult tickets were sold and 8 student tickets were sold.
How did I get this? 

First, let's see what information is given:
12 tickets sold
(S)tudent ticket = $9
(A)dult ticket = $12
Total cost = $120

So, we know student tickets plus adult tickets total up to $12. We can create an equation out of this: S + A = 12

And we know $9S + $12A = $120 

The variables represent the amount of tickets sold, which is unknown. 
A = 12 - S , substitute this into the second equation.

9s + 12(12-s) = 120
Distribute the 12 into the parenthesis.

9s + 144 - 12s = 120
Combine like terms

-3s + 144 = 120
Subtract both sides by 144, left side cancels out.

-3s = -24
s = -24/-3
s = 8 

So, 8 student tickets were sold. Plug the value of s into our first equation.

A = 12 - 8 
A = 4 (adult tickets)