Alyssa had 87 dollars to spend on 6 books. After buying them she had 15 dollars . How much did each

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Alyssa had 87 dollars to spend on 6 books. After buying them she had 15 dollars . How much did each book cost ?

Let b be the cost of each book. We're given:
87 - 6b = 15

Typing this equation into search engine, we get:
b = 12