Answer: decrease by $500 million.
Step-by-step explanation:
Money supply is the total amount of money that are in circulation at a particular period of time. 
From the question,
Required reserve ratio = 10% 
Open market sale = $50 million 
Money multiplier = 1/10%
= 1/0.1 = 10
Money supply = $50 million × 10
= $ 500 million
Since the FOMC orders an open market sale, this will lead to the decrease by $500 million.