Problem Solving

Hello gmat_mission.

This is how I'd solve it:

We need to find (x+y+z)/3.

Since
x + z = 8
2y + z =11

Then, we can get that x-2y=-3. If we derive x we will get: x=2y-3.

Now, we substitute it in the equation 3x + y= 19, hence $$3\left(2y-3\right)+y=19\ \Rightarrow\ \ 6y-9+y=19\ \Rightarrow\ \ 7y=28\ \Rightarrow\ \ y=4.$$ Therefore $$x=2\left(4\right)-3=5\ \ \ and\ \ \ \ 5+z=8\ \Rightarrow\ \ z=3.$$ This way we get that $$Average=\frac{5+4+3}{3}=\frac{12}{3}=4.$$ Therefore, the correct answer is the option [spoiler]C=4[/spoiler].

I hope it helps.