Question

If y=15 when x=5, find y when x=7, given that y varies directly with x.

Answer 1

when x = 7, y = 21

Explanation:

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.

y = 3x

y = 3(7)

y = 21