A, B, and C working alone can finish constructing a house in 6, 12, and 18 hours respectively. A starts alone and works for 1 hour. He is then joined by B and they work together for 1 hour. Finally C joins them and they all work together to finish the work. If they start working at 9 am, at what time will they finish?  Choose the correct choice:  A) 12:05 pm  B) 12:15 pm  C) 12:35 pm  D) 12:55 pm  E) 1:15 pm
A completes 1/6 of the job each hour.   B completes 1/12 of the work each hour.   C completes 1/18 of the work each hour.   A completes 1/6 of the work in the first hour.   Working together, A and B complete another 3/12 for a total of 5/12.   Working together, A, B, and C can complete 11/36 of the work each hour.   To complete 7/12, the time required is (7/12) / (11/36) = 21/11 hours.   The work will be completed at 12:00 plus 10*60/11 minutes which is about 12:55 pm.   The answer is 12:55 pm.
<result>D</result>
A, B, and C working alone can finish constructing a house in 6, 12, and 18 hours respectively. A starts alone and works for 1 hour. He is then joined by B and they work together for 1 hour. Finally C joins them and they all work together to finish the work. If they start working at 9 am, at what time will they finish?