Machine A produces 20 units in 5 hours. Machine B produces 30 units in 6 hours. If both machines work together, how many units can they produce in 10 hours?

✓ Correct Answer: Option C

Machine A's rate is 20/5 = 4 units/hour. Machine B's rate is 30/6 = 5 units/hour. Together, their combined rate is 4+5 = 9 units/hour. In 10 hours, they produce 9 * 10 = 90 units.