The formula for simple interest, where A = the final value, P = the principal (original) amount, r = interest rate per period, and t = number of time periods, is A = P(1 + rt).
Example: If Susan borrows $4500 at a 10% annual interest rate, in 5 years, how much will she owe if she hasn’t made any payments? Answer: A = 4500(1 + 0.10 × 5) = $6750.