Question 1204843
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The response from the other tutor shows a typical solution using the standard formal algebraic method.<br>
If formal algebra is not required, there is an easy informal method for solving any 2-part mixture problem like this.<br>
The desired percentage of 30 is 1/4 of the way from 10 to 90.  (from 10 to 90 is a difference of 80; from 10 to 30 is a difference of 20; 20/80 = 1/4.)<br>
That means 1/4 of the mixture is the 90% alcohol.<br>
1/4 of 100ml is 25 ml.<br>
ANSWER: The mixture should be 25ml of the 90% alcohol and 75ml of the 10% alcohol.<br>