Answer:
$3 for 1 book
Step-by-step explanation:
if 3 books are $9, then to find out how much only one of them costs you have to divide the money by the number of things you're buying. So if you divide 9 by 3 you get 3, so 1 book costs $3.
This also checks because if you buy one book for $3 but want to buy 3 books, you have to pay thrice the amount for the books, so you would have to pay $9 because 3 times 3 is 9
