It takes 7 high school students, working at identical constant individual rates, 10 hours to paint a certain house. At what time will the house be fully painted if 7 students start painting at 9:00 am and one student, working at the same rate, is added per hour starting at 5:00 pm? Pick one:
A) 5:40 pm
B) 6:00 pm
C) 6:20 pm
D) 6:40 pm
E) 7:00 pm
Each student paints at a rate of 1/70 of the house per hour.   In 8 hours, the 7 students can paint 56/70 of the house.   From 5 pm to 6 pm, 8 students paint another 8/70 for a total of 64/70.   9 students can paint the remaining 6/70 of the house in 6/9 of an hour = 40 minutes.   The house is completed at 6:40 pm.   The answer is 6:40 pm.
<result>D</result>
It takes 7 high school students, working at identical constant individual rates, 10 hours to paint a certain house. At what time will the house be fully painted if 7 students start painting at 9:00 am and one student, working at the same rate, is added per hour starting at 5:00 pm?