The MATH behind the interest rate.For calculating option and future prices one uses socalled riskfree interest rates r, guaranteed interest you can collect without (reasonable) financial dangers involved. An annual interest of 6% means you start with e.g. $100 and after a year you have $100 + 6% of $100 = $106 . For the option prices you apply continuously compounded interest  you let the money work and collect interest on the interest, again interest on that interest and so on. The formula for that is
Money after one year =
Money after one year = $100 * exp(0.06) = $106.18 The result is similar, somewhat higher because of the additional interest on the interest that was added up.
