Amortized loans – declining balance, annuity, and the flat-rate trap

MathematicsSequences & Financial MathAges 17–18

Enter the loan amount, annual interest rate, and term in months, then choose a repayment method: equal principal with interest on the declining balance, equal monthly payments (annuity), or flat interest calculated on the original principal. The chart splits each month's payment into principal and interest, plots the declining balance, and shows total interest — revealing that the real interest rate behind a "flat 1%/month" offer is nearly double the advertised figure.