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 30% by volume and the capacity of the tank is 24 gallons. If the water evaporates from the tank at the rate of 0.4 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?
Choose the correct answer
A) 30 %  B) 35 %  C) 40 %  D) 45 %  E) 50 %

Rationale: "the number of gallons in the tank is ( 1 / 4 ) 24 = 6 gallons the amount of sodium chloride is 0.3 ( 6 ) = 1.8 gallons at the start , the amount of water is 0.7 ( 6 ) = 4.2 gallons after 6 hours , the amount of water is 4.2 - 0.4 ( 6 ) = 1.8 gallons the concentration of water is 1.8 / ( 1.8 + 1.8 ) = 1.8 / 3.6 = 50 % the answer is e ."
Answer: E