Question 970723
That is 4 doubling times or 18*4=72 years

P=Po e^rt
P=Principal at end
Po=Principal at beginning
P/Po=e^rt
ln (P/Po)=rt
But P=2 Po
ln 2=rt
0.693=rt
69.3%=rt
We round it to 72, because it is divisible by so much more
72/4 years =18 years

In this problem
ln (16)=.04t
t=69.31 years.  The rule says 72 years, so that is the answer.