Three friends bought 4, 6, and 8 books. The table shows the cost of the books. What is the constant of proportionality? 2 7.5 15 30

Your anwser would be 8

The answer is 15 because if you look at the number if books, it goes by 2. If you look at the prices, it goes by 15. What it is mainly asking for is what the number goes by. Therefore the correct answer is 15.