When you hear or see the word “per” in math, think of the question/word problem telling you to find the “for each”. This basically mean you’re dividing. So, let’s divide your numbers from each other.
I'd do 9x4=36 because the 9 is from your money and the 4 is from your books. Now since you want to find out how much 4 books cost you just multiply that by 9
