This is a nice question on exponential growth. It shows how quickly something can grow if it doubles each day.
We know that the dish is fully covered with bacteria by Day 4. Now if we were to back track, then on Day 3, half of the Petri dish would have been covered. And then on Day 2, a quarter of the dish would be covered with the bacteria. So the answer is on Day 2.
This problem can be solved in your head, but with Omerus you can label each line item for a better explanation. This is useful if you were to share your answers using the Share button in Omerus.
Petri dish is completely full of bacteria on fourth day (1 represents 100%)
Fourth day amount / daily growth rate = amount of bacteria on third day
Third day amount / daily growth rate = amount of bacteria on second day