Answer:
 9.0 years
Step-by-step explanation:
The exponential equation for the population can be written as ...
 population = (initial population)(1 +growth rate)^t
 p = 24000(1.13^t)
We want this to be 72000, so ...
 72000 = 24000(1.13^t)
Dividing by 24000 gives ...
 3 = 1.13^t
Taking logs, we have ...
 log(3) = t·log(1.13)
 log(3)/log(1.13) = t ≈ 8.98
It will take about 9.0 years for the elk population to reach 72,000.
