SOLUTION: You have 80 ml of a 85% alcohol solution. How many mls of a 15% solution must be added to make a 40% solution?

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Question 981444: You have 80 ml of a 85% alcohol solution. How many mls of a 15% solution must be added to make a 40% solution?
Found 2 solutions by macston, josmiceli:
Answer by macston(5194) About Me  (Show Source):
You can put this solution on YOUR website!
.
Let x=ml of 15% solution
.
(0.85)(80ml)+(0.15)x=(0.40)(x+80ml)
68ml+0.15x=0.40x+32ml Subtract 32 ml from each side.
36ml+0.15x=0.40x Subtract 0.15x from each side.
36ml=0.25x Multiply each side by 4.
144ml=x

Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +a+ = ml of 15% solution needed
+.15a+ = ml of alcohol in 15% solution
+.85%2A80+=+68+ ml of alcohol in 85% solution
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+%28+.15a+%2B+68+%29+%2F+%28+a+%2B+80+%29+=+.4+
+.15a+%2B+68+=+.4%2A%28+a+%2B+80+%29+
+.15a+%2B+68+=+.4a+%2B+32+
+.25a+=+36+
+a+=+144+
144 ml of 15% solution are needed
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check:
+%28+.15a+%2B+68+%29+%2F+%28+a+%2B+80+%29+=+.4+
+%28+.15%2A144+%2B+68+%29+%2F+%28+144+%2B+80+%29+=+.4+
+%28+21.6+%2B+68+%29+%2F+224+=+.4+
+89.6+=+.4%2A224+
+89.6+=+89.6+
OK