Question 1126501
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Here is a quick and easy alternative to the standard algebraic method for solving mixture problems like this, as shown by the other tutors.<br>
You are adding 100% pure acid to the 10% acid; the percentage of the mixture moves from 10% towards 100%.  You stop when the percentage is 64%.<br>
(1) 64% is 3/5 of the way from 10% to 100%.  (Do a little mental arithmetic: 100-10 = 90; 64-10 = 54; 54/90 = 3/5)
(2) That means 3/5 of the mixture must be the 100% pure acid you are adding.<br>
3/5 of 90 L is 54 L.  So 54 L of the 100% acid and 36 L of the 10% acid.