SOLUTION: A 18 gallon salt-water solution contains 15% pure salt. how much pure salt should be added to produce the solution to 20% salt?

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Question 830613: A 18 gallon salt-water solution contains 15% pure salt. how much pure salt should be added to produce the solution to 20% salt?
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
Best judgement about this is that an estimate is possible, assuming 18 gallons of water as "pounds" of water and that 15% of this value is the amount of salt dissolved in it.

You must use the density of water, 8.345 pounds per gallon; or whatever value you can best accept.

18%2Agallons%288.345%2Apound%2Fgallon%29=2703.78 pounds

Next, assuming that concentration is MASS per MASS, we can continue...

%28%280.15%29%282704%29%2B1.00%2Am%29%2F%282704%2Bm%29=0.20, using m for the mass of salt to add, in "pounds".
Solve for m.