Answer:
$8 each
Explanation:
We know Sally has $77 to spend. She bought 8 books, and was then left with $13. First, let's subtract 13 from 77 to see how much it cost in total.
77 - 13 = 64
Now, to find out how many each book cost, we have to divide the total amount by how many books Sally got.
64 / 8 = 8
Each book cost $8.