Answer:
17.3 years
Step-by-step explanation:
For some annual interest rate r and time in years t, the initial account value is multiplied by e^(rt).
You want the multiplier e^(.04t) to have a value of 2:
2 = e^(.04t)
ln(2) = 0.04t . . . . . take natural logs
t = ln(2)/0.04 = 17.329 . . . . . divide by the coefficient of t
It will take about 17.3 years for the investment to double.
