60 minutes in an hour. 4 sets of 15 in 1 hour. 120 times 3 is 360. That's 1 set of 15 minutes. 360 times 3 is 1080. That's the second set of 15 minutes. 1080 times 3 is 3240. That's the third set of 15 minutes. 3240 times 3 is 9720. That's the 4th set of 15 minutes.
9720 is the amount of bacteria that will be in your Petri dish at the end of 1 hour.
