This is about direct variation. If y varies directly with x, then there is a constant factor of variation, k. The constant of variation can be found by ʸ⁄ₓ
k = ʸ⁄ₓ
k = ¹⁵⁄₅ = 3
Now that we have found k, we simply multiply x by it to find y (this is because y is directly proportional to x). We plug in our x = 7 to find y at this point.
Firstly, you do 90 - 42 to get the amount of brownies sold, which is 48. Then, you put 42 and 48 into a ratio like 42:48. After that, all you need to do is simplify to get 7:8.
