Question 1133016
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Most people who write math problems are not chemists... so let's treat this as a mixture problem and ignore the fact that the stated concentrations are physically impossible.<br>
Here is the easiest way to solve a "mixture" problem like this involving two ingredients....<br>
(1) The desired concentration, 25%, is 1/4 of the way from 20% to 40%.  (25-20 = 5; 40-20 = 20; 5/20 = 1/4)<br>
(2) That means 1/4 of the mixture is the 40% solution; 1/4 of 12 liters is 3 liters.<br>
ANSWER: 9 liters of the 20% solution and 3 liters of the 40% solution.<br>
For comparison, here is the traditional algebraic approach, using an equation that says the amount of salt in the two ingredients has to be equal to the amount in the mixture:<br>
{{{.20(x)+.40(12-x) = .25(12)}}}
{{{20(x) + 40(12-x) = 25(12)}}}
{{{20x+480-40x = 300}}}
{{{180 = 20x}}}
{{{x = 9}}}<br>
So x = 9 liters of 20% and (12-x) = 3 liters of 40%.<br>
The first method gets you to the answer faster and with far less work than the second....