A bookstores sells frequent buyer Discount cards For $12 each. The cost of each book with the discount card is $7. Cost of a book without the card is $9. After how many book purchases does a card holder in a non-card holder spend the same amount of money, if you include the cost of the card?

6 book purchases.

Step-by-step explanation:

As the bookstore sells frequent buyers discount cards at $ 12, which make the books that usually cost $ 9 cost $ 7, to determine from how many books a card-holder and a common buyer will spend the same amount of money is necessary perform the following calculation:

9 - 7 = 2

12/2 = 6

Therefore, after purchasing 6 books, both categories of buyers will have spent the same amount of money. This is verified with the following calculation:

Card holder = 12 + 7 x 6 = 54

Non-card holder = 9 x 6 = 54

it would rotate about 180 degrees.