Three pipes A, B and C can fill a tank in 7 hours. After working at it together for 2 hours, C is closed and A and B can fill the remaining part in 8 hours. How many hours will take C alone to fill the tank?  Select:
A) 35/8  B) 38/7  C) 43/6  D) 49/4  E) 56/3
The rate of A+B+C is 1/7 of the tank per hour.   After 2 hours, the tank is 2/7 full.   The rate of A+B is 5/7 * 1/8 = 5/56 of a tank per hour.   The rate of C is 1/7 - 5/56 = 3/56   C can fill the tank in 56/3 hours.   The answer is 56/3.
<result>E</result>
Three pipes A, B and C can fill a tank in 7 hours. After working at it together for 2 hours, C is closed and A and B can fill the remaining part in 8 hours. How many hours will take C alone to fill the tank?