Task: Question Answering
Input (English): Problem: pipe a can fill a tank in 6 minutes and pipe b cam empty it in 24 minutes . if both the pipes are opened together after how many minutes should pipe b be closed , so that the tank is filled in 30 minutes ? 
Options: a ) 18 , b ) 27 , c ) 96 , d ) 27 , e ) 21
Output (English): c