Machine A can complete a task in 3 hours. Machine B can complete the same task in 6 hours. If both machines work together at their constant rates, how many hours will it take them to complete the task?

✓ Correct Answer: Option B

Machine A's rate is 1/3 task per hour. Machine B's rate is 1/6 task per hour. Their combined rate is (1/3) + (1/6) = (2/6) + (1/6) = 3/6 = 1/2 task per hour. To complete 1 task, the time required is 1 / (1/2) = 2 hours.