First, we will make a list of the bacteria growth during each hour.
Start: 15
1st hour: 30
2nd hour: 60
3rd hour: 120
4th hour: 240
5th hour: 480
6th hour: 960
7th hour: 1920
8th hour: 3840
9th hour: 7680
10th hour: 15360
11th hour: 30720
12th hour: 61440
As you can see, the number of bacteria present is doubled at the end of each hour, as the question states.
However the question refers to the number of bacteria that grow in the twelfth hour and not the total number of bacteria in the twelfth hour.
So the answer will be the subtraction between the bacteria in the twelfth hour minus the bacteria in the eleventh hour.

In conclusion, the answer is 30720