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 4 hours ?
Rationale: "the number of gallons in the tank is ( 1 / 4 ) 24 = 6 gallons the amount of sodium chloride is 0.4 ( 6 ) = 2.4 gallons at the start , the amount of water is 0.6 ( 6 ) = 3.6 gallons after 4 hours , the amount of water is 3.6 - 0.5 ( 4 ) = 1.6 gallons the concentration of water is 1.6 / ( 2.4 + 1.6 ) = 40 % the answer is a ."
annotated_formula: multiply(divide(subtract(divide(multiply(4, subtract(const_100, 40)), const_100), multiply(0.5, 4)), subtract(4, multiply(0.5, 4))), const_100)