Question

Paperback book cost $0.50
Hardback book cost $0.75 each

If 12 books were purchased for a total of $6.75, how many paperback books were purchased?

Answer 1

9 paperback books were purchased

Answer 2

9 paperbacks will be sold since 
 9 x 0.50 = 4.50
Because you bought 9 books you have to buy 3 more hardback books so:
3 x 0.75 = 2.25
2.25 + 4.50 = 6.75