Joint Variation (Jump to: Lecture | Video )

Joint Variation
Joint variation is when one quantity varies directly as the product of at least two other quantities. y varies jointly as x and z, when there is some nonzero constant, k.

Suppose y varies jointly as x and z. What is y when x = 2 and z = 3, if y = 20 when x = 4 and z = 3?

First, we plug in our known values to solve for the unknown constant, k:

Figure 1.

Now that we know k, we can solve for the unknown y:

Figure 2.