One pack of strings is $14. This is because 72-16=56. I subtracted the 16 because it is how much the book cost, so it does not have anything to do with the amount one pack of strings cost. Then, I divided 56 by 4 which equals 14. I divided 56 by 4 because the question is how much one pack is, and Noah and Kate bought a total of 4 packs. This proves that one pack of strings costs $14.
Hope this helps you!
