A can finish a job in 12 hours, while B can finish it in 18 hours. If they work together for 6 hours, how much of the job is left?
Answer: B. 1/3 of the job
A's rate is 1/12 and B's rate is 1/18. Together, they complete 1/12 + 1/18 = 5/36 of the job per hour. In 6 hours, they complete 30/36 = 5/6 of the job, leaving 1/6.