# 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?

May 2, 2017

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 $\frac{y}{x} = k$.

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

$\frac{15}{-} 6 = \frac{y}{8}$.

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