Task: Question Answering
Input (English): Problem: a tank is filled to one quarter of its capacity with a mixture consisting of water and sodium chloride . the proportion of sodium chloride in the tank is 40 % by volume and the capacity of the tank is 24 gallons . if the water evaporates from the tank at the rate of 0.5 gallons per hour , and the amount of sodium chloride stays the same , what will be the concentration of water in the mixture in 6 hours ? 
Options: a ) 18 % , b ) 20 % , c ) 22 % , d ) 24 % , e ) 26 %
Output (English): b