Tally allows you to calculate interest in two ways: in easy mode and in advanced mode. In most cases, interest is computed on past-due bills or late payments, as well as on loans. Interest is a source of income for the recipient and a source of expense for the payment.
SI = P R T, where P = Principal, R = Rate of Interest, and T = Time Period, is a formula for calculating simple interest.
Simple interest is found in monthly amortised car loans and monthly amortised retailer instalment loans; as the loan balance decreases with each monthly payment, so does the interest. CDs pay a certain amount of interest on a predetermined date, reflecting simple interest.