Machine A can produce 100 units in 5 hours, and Machine B can produce 100 units in 4 hours. If both machines work together at their constant rates, how many hours will it take them to produce 450 units?
✓ Correct Answer: Option C
Machine A's rate is 20 units/hour, and Machine B's rate is 25 units/hour. Their combined rate is 45 units/hour. To produce 450 units, it will take 450 / 45 = 10 hours.