Question 1202656
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The formal algebraic solutions in the responses from the other tutors are fine.<br>
But as one of the tutors notes at the end of his response, the formal algebra is not needed.  Common sense tells us that, since 13% is exactly halfway between 9% and 17%, the amount of 9% saline should be equal to the amount of 17% saline.<br>
That tutor implies that solving the problem by that method only works when the percentage of the mixture is halfway between the percentages of the two ingredients.<br>
But that is not the case.  That informal method can be used to solve any 2-part mixture problem quickly and easily.  In this problem, the percentage of the mixture is 1/2 of the way from the 9% to the 17%, so half of the mixture is the 17% saline.  If in a similar problem the percentage of the mixture is 2/5 of the way from the lower percentage to the higher percentage, then 2/5 of the mixture would have to be the higher percentage ingredient.<br>
You should be able to search this website for many examples of problems like this for which I have provided responses showing solutions by this quick informal method.<br>