A person borrowed $$21000$ from a bank at an interest rate of $6\%$ compounded monthly. The loan will be repaid in $48$ equal monthly installments over $4$ years. What is his monthly payment amount? Immediately after his $20$th payment, Bob desires to pay off the remainder of the loan in a single payment. What is this remaining balance?

Monthly Payment $=$$

Single payment for remaining Balance after $20$th payment $=$ $