In a favorable environment, bacteria can reproduce about every 20 minutes. There are four bacteria that is left out at room temperature. After two hours, how many bacteria

Every time they reproduce, their number doubles.

There are 120 minutes in 2 hours.

So, they reproduced 120/20=6 times.

In the end, there are 4*2*2*2*2*2*2=256 bacteria.

Here is a chart:

After 0 hours and 0 min.:

There are 4 bacteria

After 0 hours and 20 min.:

There are 8 bacteria

After 0 hours and 40 min.:

There are 16 bacteria

After 1 hours and 0 min.:

There are 32 bacteria

After 1 hours and 20 min.:

There are 64 bacteria

After 1 hours and 40 min.:

There are 128 bacteria

After 2 hours and 0 min.:

There are 256 bacteria