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