A population is currently 400 and growing at a rate of 6% per year.
If the population continues this trend, how many full years does it take to at least double?
number = 400(1.06)^n , n is the number of years
400(1.06)^n ≥ 800
1.06^n ≥ 2
take logs
nlog1.06 ≥ log2
n ≥ log2/log1.06 ........
Interesting sideline: look up "rule of 72" and see how it applies to this question.