SOLUTION: how much pure acid should be mixed with 3 gallons of a 60% acid solution in order to get 90% acid solution

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Question 501139: how much pure acid should be mixed with 3 gallons of a 60% acid solution in order to get 90% acid solution
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
In words:
( gallons of acid you end up with ) / ( total gallons of solution ) = 90%
Let +a+ = gallons of pure acid to be added
given:
+.6%2A3+=+1.8+ = gallons of acid in 60% solution
+a+%2B+3+ = total gallons of solution
-------------
+%28+a+%2B+1.8+%29+%2F+%28+a+%2B+3+%29+=+.9+
+a+%2B+1.8+=+.9%2A%28+a+%2B+3+%29+
+a+%2B+1.8+=+.9a+%2B+2.7+
+.1a+=+.9+
+a+=+.9%2F.1+
+a+=+9+
9 gallons of pure acid need to be added
check answer:
+%28+a+%2B+1.8+%29+%2F+%28+a+%2B+3+%29+=+.9+
+%28+9+%2B+1.8+%29+%2F+%28+9+%2B+3+%29+=+.9+
+10.8+%2F+12+=+.9+
+.9+=+.9+
OK