The doubling period of a baterial population is $15$ minutes. At time $t = 80$ minutes, the baterial population was 60000. For some constant A, the formula for the population is $p(t)=A e^{kt}$ where $\displaystyle k=\frac{\ln 2}{15}$. What was the initial population at time $t = 0$?
Find the size of the baterial population after 5 hours.