Question 511467
You need 5 liters total.  This needs to be 25.6% 'pure' acid.
.256*5 = 1.28 liters of pure acid in the 5 liters total solution.
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The amount of the 40% solution is:  x
so, the amount of the 16% solution is: 5-x
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.40x + .16(5-x) = 1.28
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multiply by 100 to eliminate the decimal
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40x + 16(5-x) = 128
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40x + 80 -16x = 128
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24x = 48
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x = 2
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so
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5-x = 3
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Check to see if the amount of pure acid is correct.
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.4*2 + .16*3 = 1.28
which is exactly the amount of pure acid calculated at the start.
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Answer:  Mix 2 liters of 40% acid and 3 liters of 16% acid to make 5 liters of 25.6% acid.
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Done.