The question is missing a few assumptions there: if the Fed's reserve requirement is 20% and the banks are not holding extra reserves, then the Multiplier can be calculated as:
1/20%
= 1/20 *100
= 5
Sp a $5 Million buy will increase the money supply by $25 Million as the maximum.
If the reserve requirement is different or banks hold extra reserves, then the answer will be different.
