Machine P can complete a task in 10 hours. Machine Q can complete the same task in 15 hours. If both machines work together, how many hours will it take to complete 3/4 of the task?

✓ Correct Answer: Option B

Machine P's rate is 1/10 task/hour, Q's is 1/15 task/hour. Combined rate is 1/10 + 1/15 = 3/30 + 2/30 = 5/30 = 1/6 task/hour. Time to complete 3/4 task = (3/4) / (1/6) = 3/4 * 6 = 18/4 = 4.5 hours.