SOLUTION: How many liters of pure acid must be mixed with 20 liters of a 15% acid solution to get a 20% mixture?

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Question 345458: How many liters of pure acid must be mixed with 20 liters of a 15% acid solution to get a 20% mixture?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let x = liters of pure acid needed
In words:
(total liters of acid)/(total liters of solution) = 20%
given:
acid already in solution: .15%2A20+=+3 liters
-------------
%28x+%2B+3%29%2F%2820+%2B+x%29+=+.2
x+%2B+3+=+4+%2B+.2x
.8x+=+1
x+=+1.25 liters
1.25 liters of acid are needed
check:
%28x+%2B+3%29%2F%2820+%2B+x%29+=+.2
%281.25+%2B+3%29%2F%2820+%2B+1.25%29+=+.2
4.25%2F21.25+=+.2
4.25+=+.2%2A21.25
4.25+=+4.25
OK