Joe is studying a bacteria population.  There are 20 bacteria present at 3:00 p.m. and the population doubles every 3 minutes.  Assuming none of the bacteria die, how many bacteria are present at 3:15 p.m. the same day?
We first need to find out how many minutes have passed since 3:00 p.m. Yes, it is now 3:15 p.m., so 15 minutes have passed. Right. And we are told that the bacteria population doubles every 3 minutes. That means that at 3:03 p.m., there were 40 bacteria. And at 3:06 p.m., there were 80 bacteria. We can keep going until we reach 3:15 p.m. Right. So at 3:09 p.m., there were 160 bacteria. And at 3:12 p.m., there were 320 bacteria. Finally, at 3:15 p.m., there were 640 bacteria. Wow, that bacteria is multiplying quickly!

