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?

1 Answer
May 2, 2017

Solve a proportion.


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.