Task: Question Answering
Input (English): Problem: two pipes p and q can fill a cistern in 10 and 15 minutes respectively . both are opened together , but at the end of 4 minutes the first is turned off . how many more minutes will it take for the cistern to fill after the first pipe is turned off ? 
Options: a ) 3.5 , b ) 4 , c ) 4.5 , d ) 5 , e ) 5.5
Output (English): d