Question

Mike had 315 to spend on 9 books. After buying them he had 18 dallara. How much did each book cost?

Answer 1

Cost each book: C=?
C=(315-18)/9
C=(297)/9
C=33

Answer:
Each book cost 33

Answer 2

Remark
Let the cost of each book = x
9*x + 18 = 315     Subtract 18 from each side
9x + 18 - 18 = 315 - 18
9x = 297    Divide both sides by 9
9x/9 = 297 / 9
x = 33

Conclusion
Each book cost 33 dollars.

A small point. 
You really don't have to offer this many points. 15 or even 10 will get someone's attention.