You can put this solution on YOUR website! What quantity x of a 75% acid solution must be mixed with a 25% solution to produce 500 mL of a 40% solution?
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s = 75% acid volume
t = 25% acid volume
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s + t = 500
0.75s + 0.25t = 0.4*500 = 200 --> x4
3s + t = 800
1s + t = 500
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2s = 300
s = 150 ml of 75%
t = 350 ml of 25%