1. The problem statement, all variables and given/known data A 6-gallon radiator is filled with a 40% solution of antifreeze in water. How much of the solution must be drained and replaced with pure antifreeze to obtain a 65% solution? The answer is given: Ans. 2.5 gallons 3. The attempt at a solution Let x be the amount of added pure antifreeze (which is equal to the amount drained from the original solution (or so I presume)). Thus I reason 40/100*(6) is the amount of antifreeze in the orig. solution 60/100*(6) water 40/100*(6-x) is the amount of antifreeze after the drainage 60/100*(6-x) water 65/100*(6) is the amount of antifreeze in the new solution 35/100*(6) water I recon that 40/100*(6-x)+60/100*(6-x)+65/100*(6) should be equal to the new full tank, so 6. But after the calculation i get 3,9, not 2,5. I would be interested in the reasoning here especially, if someone will bother.