Answer:
A) if the MPC = 0.8, then the MPS = 0.2, and the multiplier = 1 / 0.2 = 5
if investment increases by $30 billion, aggregate demand will increase by $150 billion ($120 billion in consumption and $30 billion in investment).
B) if Mountainia's MPS = 0.25, then the multiplier = 1 / 0.25 = 4
if $5 billion in investments are postponed, then the aggregate demand will decrease by $20 billion ($15 billion in consumption and $5 billion in investment)