If $2x - y = 8$ and $x - 2y = 4$, then $x + y =$ —

Solving the system: from $x - 2y = 4$, $x = 4 + 2y$, so $2(4+2y) - y = 8 \Rightarrow 3y = 0 \Rightarrow y = 0$ and $x = 4$. Hence $x + y = 4$.