Question 981311
P=8000e^rt
16000=8000e^rt
2=e^(.07)t
ln of both sides
0.693=0.07*t
9.9 years.  This is consistent with the rule of 72 (easier to work with), which is better as the rule of 70.
Divide the interest rate (%) into 70, and one gets the doubling time in years.