Machine A can complete a certain job in 6 hours working alone. Machine B can complete the same job in 10 hours working alone. If both machines work together at their constant rates for 2 hours, what fraction of the job will they complete?
✓ Correct Answer: Option C
Machine A's rate is 1/6 job/hour. Machine B's rate is 1/10 job/hour. Together, their rate is (1/6 + 1/10) = 8/30 = 4/15 job/hour. In 2 hours, they complete 2 * (4/15) = 8/15 of the job.