What is the average (arithmetic mean) of 3x and 15z?

(1) x + 5z = 20
(2) x + z = 8


My question:
I do not understand how you can figure our the first value but not the
second... Isnt it set up the same

If we multiply everything in (1) by 3, we will get the sum for 3x + 15z = 60. So then we can simply divide 60 /2 = 30 to get the avg. If we multiply (2) by 3 we get 3x + 3z = 24, we are not interested in 3x and 3z, we are interested in 3x and 15z. Similarly, if we multiply (2) by 15 we get 15x + 15z = 120. Again, that tells us nothing about the sum of 3x and 15z.

Take care.