The exponential growth can be expressed as
y0 is the initial value
k is the growth rate
t is the time period
y is the final value
If you write the final value of the population as a function of the time, for k=4 and y0=12 the formula is
t is the determined time period the population grows, if it cuadruples every 3 years, then
first 3 years is t=1
6 years is t=2
9 years is t=3
12 years is t=4
15 years is t=5
For this population