Final answer:
The value of z varies jointly with x and y. Using the formula for joint variation, we can find the value of z when x = 2 and y = 68.
Step-by-step explanation:
Given that the value of z varies jointly with x and y, we can use the formula for joint variation: z = kxy, where k is the constant of variation. To find the value of k, we can plug in the given values and solve for k: 4 = 10k(20). Solving for k, we get k = 1/50. Now we can use this value of k to find the value of z when x = 2 and y = 68: z = (1/50)(2)(68) = 2.72. Therefore, the value of z when x = 2 and y = 68 is 2.72 (option 4