Let the necessary amount of water be x. Then (1.00)(1 gallon antifreeze) = 0.40(x+1 gallon), or 1 = .40x + .40. Combining like terms: 0.60 = 0.40x.
Then x = 0.60 / 0.40 = 1.5.
Adding 1.5 gallons of water to that 1 gallon of pure antifreeze results in 2.5 gallons of 40% antifreeze.
