A, B, and C working alone can finish constructing a house in 6, 9, 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 answer:
A) 11:45 am  B) 11:55 am  C) 12:10 pm  D) 12:25 pm  E) 12:40 pm
A completes 1/6 of the job each hour.   B completes 1/9 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 5/18 for a total of 8/18.   Working together, A, B, and C can complete 6/18 of the work each hour.   To complete 10/18, the time required is (10/18) / (6/18) = 10/6 hours.   The work will be completed at 12:00 plus 4*60/6 minutes which is 12:40 pm.   The answer is 12:40 pm.
<result>E</result>
A, B, and C working alone can finish constructing a house in 6, 9, 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?