Here is a list showing how the bacteria grows:
Begin - 15
1st hour - 30
2nd hour - 60
3rd hour - 120
4th hour - 240
5th hour - 480
6th hour - 960
7th hour - 1,920
8th hour - 3,840
9th hour - 7,680
10th hour - 15,360
11th hour - 30,720
12th hour - 61,440
As you can see, the number of bacteria present is doubled at the end of each hour, as the question states. However, the question is not asking for the total number of bacteria at the end of the 12th hour; it is asking for the bacteria GROWN during the 12th hour.
Because there were 30,720 bacteria at the end of the 11th hour, and 61,440 at the end of the 12th, the number of bacteria that grew during that last hour must be 61,440 - 30,720 = 30,720 bacteria.
(P.S. the answer would be 61,440 if it was asking for the TOTAL number of bacteria at the end of the 12th hour)
