SOLUTION: How much pure acid should be mixed with 7 gallons of a 40% acid solution in order to get a 70% acid solution?

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Question 743709: How much pure acid should be mixed with 7 gallons of a 40% acid solution in order to get a 70% acid solution?
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
Pure acid would be 100%.

How much as the quantity v of pure material should be mixed with u gallons of a L% solution in order to get a T% solution?

Known: u, L, T
Unknown: v

EQUATION:
%28uL%2Bv%2A100%29%2F%28u%2Bv%29=T
uL%2Bv%2A100=Tu%2BTv
v%2A100=Tu%2BTv-uL
v%2A100-Tv=Tu-uL
v%28100-T%29=u%28T-L%29
highlight%28v=u%2A%28%28T-L%29%2F%28100-L%29%29%29

FIND VALUE FOR v:
Substitute the values from the original problem description and evaluate.