20000=15000 (1.072) ^t

20,000 = 15,000 (1.072) ^t

Divide each side by 15,000 : 4/3 = (1.072) ^t

Take 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)