Task: Question Answering
Input (English): Problem: a tank can be filled by a tap in 20 minutes and by another tap in 60 minutes . both the taps are kept open for 10 minutes and then the first tap is shut off . after this , the tank will be completely filled in what time ? 
Options: a ) 10 mins , b ) 15 mins , c ) 20 mins , d ) 25 mins , e ) 30 mins
Output (English): c