A bacteria colony doubles every hour. If it has 500 bacteria at 9 a.m., how many will it have at 2 p.m. the same day?

The counts form a geometric progression with $a=500$ and $r=2$. From 9 a.m. to 2 p.m. is 5 hours, so the count is $500\times 2^5=500\times 32=16000$. (Counting only 4 doublings gives 8000; multiplying 500 by $2\times 5=10$ hours' worth of growth is not how doubling works.)