The graph shows the relationship between the attendance at a movie premiere and the amount of money collected from the ticket sales.

N = number of compounding periods
Years = log (total / principal) / n * log (1 + rate / n)
Years = log (750 / 500) / 4 * log (1 + .025/n)
Years = log (1.5) / 4 * log (1.00625)
Years = 0.17609125906 / 4 * 0.0027058933759
Years = 0.17609125906 / 0.0108235735
Years = 16.2692348382

Source Calculator
http://www.1728.org/compint.htm