SOLUTION: How much pure acid shold be mixed with 7 gallons of a 40% acid solution in order to get a 80% acid solution?

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Question 1084679: How much pure acid shold be mixed with 7 gallons of a 40% acid solution in order to get a 80% acid solution?
Answer by josgarithmetic(39618) About Me  (Show Source):
You can put this solution on YOUR website!
%28100x%2B40%2A7%29%2F%28x%2B7%29=80
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10x%2B4%2A7=8%28x%2B7%29
10x%2B28=8x%2B56
2x=56-28
2x=28
x=14---------fourteen gallons of pure acid