It takes 6 beavers 10 hours to build a certain dam, working at a uniform rate. If two beavers start to build the same dam at 7:00 AM, and one beaver per hour is added beginning at 4:00 PM, at what time will the dam be complete?  Choose one
A) 7:30 PM B) 8:00 PM C) 9:00 PM D) 10:00 PM E) 11:00 PM
The rate of 1 beaver is 1/60.   The amount of work done by 2 beavers in 9 hours is 2/60 *9 = 18/60   In the next hour, 3 beavers do 3/60 for a total of 21/60.   In the next hour, 4 beavers do 4/60 for a total of 25/60.   In the next hour, 5 beavers do 5/60 for a total of 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 11:00 pm.   The answer is 11:00 PM.
<result>E</result>
It takes 6 beavers 10 hours to build a certain dam, working at a uniform rate. If two beavers start to build the same dam at 7:00 AM, and one beaver per hour is added beginning at 4:00 PM, at what time will the dam be complete?