The exponential growth can be expressed as
[tex]y=y_0e^{kt}[/tex]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
[tex]y=12e^{4t}[/tex]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
[tex]\begin{gathered} y=12e^{4\cdot5} \\ y=12e^{20} \\ y=5,821,982,345 \end{gathered}[/tex]