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Let the amount of old stuff drained by x gallons
so you have 4-x gallons left of 25% solution
.25(4-x) + x = .5(4)
1 - .25x + x = 2
.75x = 1
(3/4)x = 1
x = 4/3
so drain 4/3 gallon of the current solution, then top it up with pure antifreeze