Question

If f(x) varies directly with x and f(x) = 50 when x = –10, then what is f(x) when x = 15?
(Points : 4)
–150
–75
–30
–5

Answer 1

If f(x) varies directly with x and f(x) = 50 when x = –10, then what is f(x) when x = 15?

I'd suggest you use this model for direct variation f(x) = kx

Now substitute the givens: f(x) = 50 and x = -10.

Calculate the constant of proportion, k.
This problem does not mention "direct proportion," but that is the underlying concept here. You need to find the constant of proportionality, k, which Tim calls "c" (that's perfectly OK).
Once you have this, then you have a relationship between f(x) and x.
Using that relationship, you calculate f(x) for x =15 to make it easy, your answer is -5. (D)

Answer 2

thanks for helping @StudyShark2000