The question is asking you to understand that a population which is raised by a common ratio over a certain time period follows an exponential pattern. See:
After 1 hour, the population is 3*10
After 2 hours, the population is 3*3*10
After 3 hours, the population is 3*3*3*10
.....
This can be generalised as a function of t, the time in hours:
f(t) = (3^t) * 10
Since the function determines the population at a given time, it is more prudent to replace f(t) with P, the population after time t:
P = (3^t) * 10
Since 20 minutes is equal to (1/3) hours, t can be substituted for (1/3) in order to calculate the population size after 20 minutes:
P = (3^(1/3)) * 10 = 14.4224957031 ≈ 14
Therefore the population after 20 minutes is 14.