MPC = change in C / change in Y
= 90/ 150 = 0.6
k (multiplier) = 1/ (1-0.6) = 1/0.4 = 2.5
Investmetnt = 2.5 * 40 = 100
Now, Y = 300 + 100 = 400 (c)
For the first part, you can generally just memorise that MPC = change in C/ change in Y (I don't think you're ever required to go deeper into...