can I understand the average salary of $~3~$ people in this case?

Question

$$(x + y)/2 = 400$$ $$(y + z)/2 = 800$$

I need to know what is $$(x+y+z)/3$$

Answer 1

If we add two equations, we can write that $$\frac{x+y+z}{3}=800-\frac{y}{3}$$ so the average depends on the value of $y$.

Answer 2

You cannot be sure of what the average is. You only have $2$-equations with $3$-unknowns which means that there an infinite amount of $x,y,z$ that satisfy your initial equations. All of these triples are not guaranteed to have the same average. For example, the following triples satisfy your equations

$$ (400,400,1200), (300,500,1100) $$

however their averages are $\frac{2000}{3}, \frac{1900}{3}$ respectively