Question 1167703
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A typical setup for a solution using formal algebra....<br>
x ml of 8%, plus (500-x) ml of 3%, yields 500 ml of 7%:<br>
{{{x(.08)+(500-x)(.03) = 500(.07)}}}<br>
Solve using basic algebra; I leave that to you.<br>
Here is a quick way to find the answer informally.<br>
(1) 7% is 1/5 of the way from 8% to 3%.  (Picture the three percentages on a number line.  From 8 to 3 is a change of 5; from 8 to 7 is a change of 1; 7 is 1/5 of the way from 8 to 3.)<br>
(2) That means 1/5 of the mixture is the 3%.<br>
ANSWER: 1/5 of 500 ml, or 100ml, of the 3% solution; the other 400 ml of the 8% solution.<br>
CHECK:
100(.03)+400(.08) = 3+32 = 35
500(.07) = 35<br>