A culture obeys the law of uninhibited growth. If 500 bacteria are present initially and there are 800 after 1 hour, how many will be present in the culture after 5 hours? How long is it until there are 20,000 bacteria?
Done in a very similar way to the question you posted at http://www.mathhelpforum.com/math-he...on-growth.html so apply what you've learned from that thread.
For the last part, substitute P = 20,000 and solve for t (the calculation is similar to that used to find k).