Question 519782
This is an interesting variation on a typical solution problem.  The same rule of keeping track of how much 'pure' stuff still applies, but in this case we are evaporating water to increase the salt concentration.
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You have 50 oz of 12% salt solution.  That means have .12*50 = 6.0 oz of pure salt.
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You need to produce 15% salt solution.
So 6 oz of pure salt = 15% of how much?
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.15x = 6
x = 6/.15
x = 40
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That means that if you have 40 oz of solution containing 6 oz of pure salt, you have a 15% solution.
You can check this by dividing 6/40 = .15
.15 * 100 = 15%
Correct.
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You have 50 oz of solution now, and you need 40 oz of solution, so you need to evaporate 10 oz of water.
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Done.