The growth of bacteria is known to follow the law of exponential growth $N(t)=N_0 e^{k t}$. If the original size of the colony is 100 bacteria and 4 hours later it is 100,000 bacteria, how many hours after the original time will the colony number count up to $1,000,000$ bacteria?
(USA Southern Illinois University Carbondale, Final Exam, Fall 2001)
$\left[5 \mathrm{~h} 20^{\prime}\right]$