$5648.85 The formula for compound interest is A = P(1+r/n)^(nt) where A = Amount of money at end of period P = Initial principle r = Rate of interest n = number of compound periods per yet t = number of years. So let's plug the numbers into the formula A = P(1+r/n)^(nt) A = 1680(1+0.0825/2)^(2 * 15) A = 1680(1 + 0.04125)^30 A = 1680(1.04125)^30 A = 1680(3.362408) A = $5648.85