It takes 6 beavers 10 hours to build a certain dam, working at a uniform rate. If five beavers start to build the same dam at noon, and one beaver per hour is added beginning at 6:00 PM, at what time will the dam be complete? Answers:
A) 7:30 PM  B) 8:00 PM  C) 8:30 PM  D) 9:00 PM  E) 10:00 PM
The rate of 1 beaver is 1/60.   The amount of work done by 5 beavers in 6 hours is 5/60 *6 = 30/60   In the next hour, 6 beavers do 6/60 for a total of 36/60.   In the next hour, 7 beavers do 7/60 for a total of 43/60.   In the next hour, 8 beavers do 8/60 for a total of 51/60.   In the next hour, 9 beavers do 9/60 for a total of 60/60.   The dam will be completed at 10:00 pm.   The answer is 10:00 PM.
<result>E</result>
It takes 6 beavers 10 hours to build a certain dam, working at a uniform rate. If five beavers start to build the same dam at noon, and one beaver per hour is added beginning at 6:00 PM, at what time will the dam be complete?