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.
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