You have a good start. The Total Payment on a Sinking Fund Loan is the Sinking Fund Deposit + Interest on the Loan. The 50 dollars per annum is the Interest on the Loan Payment. To see the math, let's go here:
Sinking Fund Repayment of a Loan
Press Calculate Total Payment. 4 is your sinking fund rate, 1000 is your loan amount, 20 is your term, and 5 is your effective rate. This gets us a total payment of 83.58. As you can see, 50 is for your interest paid, and 33.58 is your sinking fund deposit.
Therefore, if I'm reading your question A and B right, a = 33.58 and b = 50.
However, I've seen similar questions like yours when they say amortization method, they want to calculate the payment under the amortization method. IF that is the case and I misread your question, go here for part (b) answer:
Annuity Immediate Present Value
Calculate Payment: 5 for interest rate, 1000 for loan, 20 for term. Annual Payment is 80.24. This would make sense that it is a bit lower since the interest paid on the loan under the sinking fund method is less than our loan.
Let me know if you have questions.