Algebra

The store sells fiction books for $0.75 each and non-fiction books for $1.50. Today he spent $12.00 on books on 10 books.

So let x = the number of fiction books,
and let y = the number of nonfiction books.

Since there are two variables, you need two equations:
one for the cost, and one for the number of books.

cost:
0.75x + 1.5y = 12

and number:
x + y = 10

Using elimination, try to get rid of one of the variables by adding/subtracting the equations.
You can try to eliminate y by first multiplying the 1st equation by 2 and the 2nd equation by 3, then subtracting them:)

Thank you very much Kitty<3 that helped a lot.

You're very welcome. It gets easier as you do more of these problems:)