Answer:
In order to reduce the money supply by $1 billion, the FED needs to sell $100 million in securities.
Explanation:
The total effect on the money supply is given by: money withdrawn from the economy x money multiplier
money multiplier = 1 / required rate of return = 1 / 10% = 10
effect on the economy = -$100 million x 10 = -$1 billion