Answer:
Since the number of bacteria doubles every 3 hours, after 3 hours there will be 400 x 2 = 800 bacteria. After 6 hours, there will be 800 x 2 = 1600 bacteria. After 9 hours, there will be 1600 x 2 = 3200 bacteria. After 12 hours, there will be 3200 x 2 = 6400 bacteria. Finally, after 15 hours, there will be 6400 x 2 = 12800 bacteria. Therefore, there will be 12800 bacteria at the end of 15 hours.
Explanation: