basically, you're trying to find the MPS. (or 1-MPC, but you can just find MPS straight off)
because we know that Y=C+S, the Y and S for each year goes like this:
Y 150 350 550
S 50 150 250
So every year, Y increases by 200, and S increases by 100. Therefore, your MPS is 100/200, or 0.5
Therefore, your multiplier is 1/0.5 = 2.