If I bought compact disc for $11.50 and $7.50 and bought 12 discs with a total amount of $110, how many $11.50 compact discs did I buy?

This can be solved with a simple system of equations.

Let a be the number of 11.50 discs you bought, and b the number of 7.50 discs you bought.

So:

 

11.50a + 7.50b = 110

a + b = 12

 

(the answer is 5)