Question 1162996
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Here is an alternative to the standard algebraic solution method shown by the other tutor.<br>
The 20% target is much closer to the 15% of the original solution than it is to the 90% of the solution that is being added.  So the amount of 90% acid being added should be very small compared to the 980mL of the original solution.<br>
Specifically, 20% is only 1/15 of the way from 15% to 90%.  (15 to 90 is a difference of 75; 15 to 20 is a difference of 5; 5/75 = 1/15.)<br>
That means 1/15 of the final mixture should be the 90% acid that is being added.<br>
So the 980mL of the original 15% acid is 14/15 of the final mixture; that makes 1/15 of the final mixture 980/14 = 70mL.<br>
ANSWER: 70mL of the 90% acid should be used.<br>
CHECK:
.15(980)+.90(70) = 147+63 = 210
.20(980+70) = .20(1050) = 210<br>