Benny had 90 dollars to spend on 7 books. After buying them he had 13 dollars. How much did each boo

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Benny had 90 dollars to spend on 7 books. After buying them he had 13 dollars. How much did each book cost?

Let each book cost be b. We have:
7b + 13 = 90

Typing this equation into the search engine, and you get:
b = 11