Z varies jointly with x and y.

When x = 2 and y = 3, z = 60.

What is the value of z when x = 4 and y = 9?

First, let's find "k" the constant of variation:

z = k. x. y (since z varies jointly with x and y)

60 = k. (2). (3) → 60 = 6k and k = 10, So z = 10x. y

z = 10 (4) (9) = 10 (36) and z = 360