Mary had 153 dollars to spend on 7 books. after buying them she had 13 dollars. how much did each book cost?

So let's find how much money she spent, in total, on all the seven books.

What you can do is subtract the money left from the original total money to see the difference between the two, and that difference would be the amount of money spent on 7 books.

153 - 13 = 140

So she spent 140 dollars on 7 books.

Now, you want to find how much money one book costs. To do this, you can just divide, to find how many times the 7 books goes into the 140 dollars to get the total cost.

140 / 7 = 20

So each book costed 20 dollars.