Direct variation

Question

Suppose that y varies directly with x and y = 10 when x = 20. What is y when x = 15?

Answer 1

When y varies directly as x, the y = kx, where k is the constant of variation.

We are given that y = 10 when x = 20, so we can plug those values into the above equation to determine k:

10 = k * 20

k = 0.5

so our equation becomes y = 0.5 * x

we are now asked what is y when x = 15.

to compute that just plug in 15 for x in the above equation to get y:

y = 0.5 * 15 = 7.5