If banks have a desired reserve ratio of 0.20, there is no currency drain, and Erin deposits

Question:

If banks have a desired reserve ratio of 0.20, there is no currency drain, and Erin deposits $50 of cash in her bank, calculate
a. The bank's excess reserves as soon as Erin makes her deposit.
b. The maximum amount of loans that the banking system can make.
c. The increase in total deposits when the banks have no excess reserves.