Here's a solution that involves the INPUT-OUTPUT approach.

Given: x + y = 8z
So, one possible solution to this equation is x = 7, y = 9 and z = 2

Which of the following represents the average (arithmetic mean) of x, y, and z, in terms of z?
Average = (x + y + z)/2
= (7 + 9 + 2)/2
= 18/3
= 6

So, when we INPUT x = 2, the answer to the question is 6
So, we're looking for an answer choice that OUTPUTS a 6 when we INPUT x = 2
Let's check the answer choices...

(A) 2(2) + 1 = 5. No good. We want 6. ELIMINATE
(B) 3(2) = 6. KEEP
(C) 5(2) = 10. No good. We want 6. ELIMINATE
(D) 2/3 = 2/3. No good. We want 6. ELIMINATE
(E) 3(2)/2 = 3. No good. We want 6. ELIMINATE

Answer: B

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the average of x, y, and z = (x+y+z)/3
and it is given x + y= 8z
plug on 8z ==> (8z+z)/3==> 9z/3= 3z
It is B

Best of luck