Task: Question Answering
Input (English): Problem: a chemical supply company has 60 liters of a 35 % hno 3 solution . how many liters of pure undiluted hno 3 must the chemists add so that the resultant solution is a 50 % solution ? 
Options: a ) 18 , b ) 15 , c ) 20 , d ) 24 , e ) 30
Output (English): a