Question 1149316
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Here is a simple non-algebraic method for solving "mixture" problems like this.<br>
You are starting with 55% alcohol and adding pure (100%) alcohol, stopping when you get to 90% alcohol.<br>
(1) 90% is 7/9 of the way from 55 to 100.  (55 to 90 is 35; 55 to 100 is 45; 35/45 = 7/9.)
(2) That means 7/9 of the mixture has to be what is being added.<br>
So 7/9 of the mixture is the 100% alcohol and 2/9 is the 30 mL you started with.<br>
(3) 2/9 = 30 mL --> 1/9 = 15 mL --> 7/9 = 105 mL<br>
ANSWER: Add 105 mL of pure alcohol.<br>