Machine A can produce 100 units in 4 hours. Machine B can produce 100 units in 5 hours. If both machines work together, how many hours will it take them to produce 450 units?

✓ Correct Answer: Option C

Machine A's rate: 100 units / 4 hours = 25 units/hour. Machine B's rate: 100 units / 5 hours = 20 units/hour. Combined rate: 25 + 20 = 45 units/hour. To produce 450 units: 450 units / 45 units/hour = 10 hours.