The amount of interest is the increase in the account: $1680 -1500 = $180.
Simple interest is computed by I = Prt You have I = 180, P = 1500, t = 3 and you want to find r. Put the numbers in the formula and solve. 180 = 1500*r*3 180 = 4500*r 180/4500 = r = 0.04