Question 981568
A=Ao{1+r)^n ;; r is rate, n= years
1600000000000=577000000000(1+.032)^n
Divide by 577 billion
2.77296 ; don't round yet
2.77296=(1.032)^n
ln of both sides
1.0199=n ln (1.032)
1.0199= n*0.03150
31.12 years
1985=year 0
2016 =year 31
Early 2017.
Doubling time is 72/rate in per cent or 72/3.2=22.5 years.  This is nearly tripling, so one would expect 110/rate=or just over 34 years.  The answer is consistent with this latter figure.