How much water should be added to 1 gallon of pure antifreeze to obtain a solution that is 40% antifreeze

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.