Abacteria culture is grown with a population of 45 million cells and kept under observation. the cell population is found to double every hour. what will the population be after four hours?

96 million

124 million

360 million

720 million

The answer is 720 million.

Since the population doubles every hour, we will go hour by hour and double the size:
After one hour:     45 million cells × 2 = 90 million cells
After two hours:    90 million cells × 2 = 180 million cells
After three hours: 180 million cells × 2 = 360 million cells
After four hours:   360 million cells × 2 = 720 million cells