When interest is calculated on the original principal only it is called simple interest. Accumulated interest from prior periods is not used in calculations for the following periods. In this case, the amount value A, the principal P, the period of investment t, and the annual interest rate i are related by the formula A = P(1 + it).

At what rate will $500 accumulate to $615 in 2.5 years?

how many years will 500 accumulate to 630 if the annual interest rate is 7.8%?