Answer:
See below.
Step-by-step explanation:
For a, first we calculate the credit multiplier of the economy,
Credit multiplier = 1 / reserve ratio
Credit multiplier = 1 / 0.25 = 4
This means that any change in money supply will be 4 times as much in the economy, hence to induce a change of $120 billion, the Fed will decrease the money supply by 120/4 = $30 billion. This will increase the interest rates just enough to stabilize aggregate demand.
For b, we again start by calculating the credit multiplier.
Credit multiplier = 1/0.10 = 10
Since the Fed want to stimulate investment, it needs to use an expansionary monetary policy.
The Fed thus increases the money supply by 150/10 = $15 billion.
This will have the total effect of 150 billion on the whole thus achieving the Fed's objectives.
Hope that helps.