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.