Exponential quiz answer = 11.13AM

A: The petri dish is 25% full at 11.13AM. 

After taking 2 hours and 43 minutes to get to a fairly modest 25% concentration, the bacteria completely takes over the petri dish in just an additional 2 minutes.  From 11.13 to 11.14 the concentration of bacteria doubled to 50%.  A minute later (from11.14 to 11.15) the colony had doubled in size again, completely consuming the agar.

At 11.05, just 10 minutes before the end the dish was 99.9% bacteria-free.  You’d probably need a good magnifying glass to see any bacteria; 10 minutes earlier, with a concentration of less than 1 part per million, you’d need a microscope and some luck.  CoVid-19 is infecting people with the same sort of explosive exponential growth that the bacteria moved through the agar, going from barely noticeable to catastrophic in very little time.  From CoVid-19’s perspective, people in a are just like the agar in the petri dish is to the bacteria in this question; the bacteria needs the agar to reproduce, just as the virus needs us to reproduce.

In some places CoVid-19 infections have doubled every 3 days, increasing the number of infected people by 1000% in only 10 days, and by 10,000% in 20 days.  Physical distancing decreases the speed of the spread and increases the all-important doubling time (“planking” or “flattening” the curve).  Moving from a 3-day doubling time to one of 1-week slows the time for a 10,000% increase from just 20 days to almost 2 months, easing the burden on hospitals and giving people a better chance to protect themselves and others, further lengthening the doubling time.)