lols.
well this is how i tackle these questions..
well, we know that C = 100 + 0.8Y and I = 40
the economy is in equalibrium when S = I ( or AD = AS )
S is basically opposite C (haha, this is how i remmeber it)
so,
S = -100 + 0.2Y (0.2 since, MPS + MPC = 1)
let S = I
-100 +...