Task: Question Answering
Input (English): Problem: 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 2 nd pipe fills the tank 5 hours faster than first pipe and 4 hours slower than third pipe . the time required by first pipe is 
Options: a ) 13 hours , b ) 45 hours , c ) 15 hours , d ) 12 hours , e ) 11 hours
Output (English): c