06/19/2020 (2011: Countdown Round, Problem 14)

Q: 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?

A: 640

If the bacteria doubles every 3 minutes, and 15 minutes pass (between 3:00 and 3:15), then the bacteria is multiplies by 2^(15/3) = 2^5 = 32.

20 bacteria * 32 = 640 bacteria