It takes 3 workers a total of 12 hours to build a giant machine with each worker working at the same rate. If 4 workers start to build the machine at 11:00 AM, and one extra worker per hour is added beginning at 3:00 PM, at what time will the machine be complete?	Choose the correct choice from the following choices:	A) 5:30	B) 5:45	C) 6:00	D) 6:15	E) 6:30
3 workers build 1/12 of a machine in one hour.   1 worker builds 1/36 of a machine in one hour.   In the first 4 hours, 4 workers build 4*(1/36)*4=16/36 of a machine   From 3:00 to 4:00, 5 workers build another 5/36. The total is 21/36.   From 4:00 to 5:00, 6 workers build another 6/36. The total is 27/36.   From 5:00 to 6:00, 7 workers build another 7/36. The total is 34/36.   To build another 2/36, 8 workers need (2/8)*60 minutes which is 15 minutes.   The machine is complete at 6:15.   The answer is 6:15.
<result>D</result>
It takes 3 workers a total of 12 hours to build a giant machine with each worker working at the same rate. If 4 workers start to build the machine at 11:00 AM, and one extra worker per hour is added beginning at 3:00 PM, at what time will the machine be complete?