Task: Question Answering
Input (English): Problem: pipe a can fill a tank in 12 hours . due to a leak at the bottom it takes 20 hours to fill the tank . in what time the leak alone can empty the full tank ? 
Options: a ) 18 hours , b ) 23 hours , c ) 28 hours , d ) 30 hours , e ) 40 hours
Output (English): d