Suppose that the amount of algae in a pound doubles every 6 hours. If the pond initially contains 50 pounds of algae, how much algae will be in the pond after 18 hours?
After 6 hours, the pond will contain 100 pounds of algae (50 pounds doubled).
After another 6 hours (total of 12 hours), the pond will contain 200 pounds of algae (100 pounds doubled again).
After another 6 hours (total of 18 hours), the pond will contain 400 pounds of algae (200 pounds doubled again).
Therefore, after 18 hours, the pond will contain 400 pounds of algae.