A tank is filled by 3 pipes with uniform flow. The first two pipes operating simultaneously fill the tan in the same time during which the tank is filled by the third pipe alone. The 2nd pipe fills the tank 5 hours faster than first pipe and 4 hours slower than third pipe. The time required by first pipe is	Choose the correct answer
A) 13 hours B) 45 hours C) 15 hours D) 12 hours E) 11 hours

Rationale: if first pipe fills tank in x hrs , then second pipe fills tank in x - 5 hrs and 3 rd pipe fills tank in x - 9 hrs , then as per given condition 1 / x + 1 / ( x - 5 ) = 1 / ( x - 9 ) solving it , we get x = 15 hrs answer : c
Answer: C