A short exponential quiz

Here’s a simple question that illustrates a key mathematical concept underlying the spread of CoVid-19 and the rapidity with which an infectious disease can spread even when no-one around you seems to be sick.  Check it out, and share it with friends.  Use a calculator if you want. 

A colony of 20 microscopic bacteria is added to a petri dish of nutrient agar at 8.30 AM.  The bacteria reproduce rapidly, doubling in number every minute.  After just 2 hours and 45 minutes (11.15AM) the petri dish is fully infected with almost a thousand trillion trillion trillion trillion bacteria (that’s 1 followed by 51 zeroes).

Q: At what time – to the nearest second – is the petri dish one-quarter full of bacteria?

Answer >>>>