Three pipes A, B and C can fill a tank in 4 hours. After working at it together for 2 hours, C is closed and A and B can fill the remaining part in 5 hours. How many hours will take C alone to fill the tank?  Choose the correct choice from the following answers
A) 10/3  B) 15/4  C) 20/3  D) 25/4  E) 35/6
The rate of A+B+C is 1/4 of the tank per hour.   After 2 hours, the tank is half full.   The rate of A+B is 1/2 * 1/5 = 1/10 of a tank per hour.   The rate of C is 1/4 - 1/10 = 3/20   C can fill the tank in 20/3 hours.   The answer is 20/3.
<result>C</result>
Three pipes A, B and C can fill a tank in 4 hours. After working at it together for 2 hours, C is closed and A and B can fill the remaining part in 5 hours. How many hours will take C alone to fill the tank?