If you start with a single bacterium, capable of dividing every 20 minutes, how many bacteria would you have after just 4 hours

They can multiply three times every hour, 60/20 = 3.

Because it divides, it produces 2 every time per bacteria.

This means that you have to double the values each time for 12 times, because 4 * 3 = 12. This gives the results:

1 2 4 8 16 32 64 128 256 512 1024 2048

This means there will be 2048 bacteria in the body after 4 hours.