Task: Question Answering
Input (English): Problem: two pipes p and q can fill a cistern in 12 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 ) 6 , b ) 7 , c ) 8 , d ) 9 , e ) 10
Output (English): a