$18
1) Gathering the data
Sandy: $155 on 8 books
Change: $11
2) Let's write an equation for that since we don't know how much each book cost.
155 -8x = 11 Note that we're calling books by x, and the number of it: 8
155 -8x =11 Subtract 155 from both sides
-8x = 11 -155
-8x = -144 Divide both sides by -1
8x = 144 Divide both sides by 8
x = 18
3) Hence, each book costs $18