It takes just four hours for one bacterium to become .

256 bacterium.

Explanation:

suppose a bacterium would divide once in every half an hour, this means it would be double in half an hour.

therefore, a number of the bacterium after half an hour will be 1 x 2 = 2.

After one hour, we would have doubled the number of bacterium =2 x 2

= 4.

Also, in one and a half hours, it would become 4 x 2

= 8.

In 2 hours, it would be 8 x 2 = 16.

Also in two and half hours, it would  be 16 x 2

= 32.

In three hours, it would be 32 x 2

= 64.

In three and a half hours, it would be 64 x 2

= 128.

Funally, in four hours, the number of bacterium would have been multiplied to 128 x 2

= 256.

cuz he is really really short, assuming an unorthodox and inherently unlucky configuration of elevator buttons…

explanation: