minus the repayment per unit time =-c, so the rate of change of debt is:
where k=ln(1+rt/100) when t is measured in years, and rt is the annual
interest rate in percent. Now you need to solve the DE with boundary
conditions that y(0)=L the initial loan, and y(5)=0 (equivalent to the loan
being paid off after 5 years = 60 months).