20000=15000(1.072)^t
20,000 = 15,000 (1.072)^tDivide each side by 15,000 : 4/3 = (1.072)^tTake the log of each side: log(4/3) = t log(1.072)Divide each side by log(1.072): t = log(4/3) / log(1.072) 0.12494 / 0.03019 = 4.138 (rounded)