Question

The variables x=-6 and y=15 varies directly. How do you write an equation that relates the variables and find y when x=8?

Answer 1

Solve a proportion.

Explanation:

If x and y vary directly, it means y = kx, where k is the constant of proportionality.
For any pair (x, y), we have `y/x = k`.

We have (-6, 15) as one set of values, and we have (8, y) as another -- where y is unknown. Solve the proportion:

`15/-6 = y/8`.

Finish the problem by "cross-multiplying", and solving for y. You should get y = -20.