On December 31, 2017, Oakbrook Inc. rendered services to Beghun Corporation at an agreed price of $102,049, accepting $40,000 down and agreeing to accept the balance in four equal installments of $20,000 receivable each December 31. An assumed interest rate of 11% is imputed. Prepare an amortization schedule. Assume that the effective-interest method is used for amortization purposes.