SOLUTION: Please help me figure out the answer: How much of a 70% alcohol solution and how much of a 30% alcohol solution should be mixed to make 5 liters of a 50% alcohol solution?

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Question 545910: Please help me figure out the answer: How much of a 70% alcohol solution and how much of a 30% alcohol solution should be mixed to make 5 liters of a 50% alcohol solution?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let a = liters of 70% solution needed
Let b = liters of 30% solution needed
+.7a+ = liters of alcohol in 70% solution
+.3b+ = liters of alcohol in 30% solution
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In words:
( liters of alcohol ) / ( total liters of solution ) = 50%
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given:
(1) +a+%2B+b+=+5+
(2) +%28+.7a+%2B+.3b+%29+%2F+5+=+.5+
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(2) +.7a+%2B+.3b+=+2.5+
(2) +7a+%2B+3b+=+25+
Multiply both sides of 1) by 3%7D%7D+and%0D%0Asubtract+%281%29+from+%282%29%0D%0A%282%29+%7B%7B%7B+7a+%2B+3b+=+25+
(1) +-3a+-+3b+=+-15+
+4a+=+10+
+a+=+2.5+
and, since
(1) +a+%2B+b+=+5+
(1) +2.5+%2B+b+=+5+
(1) +b+=+2.5+
2.5 liters of 70% solution are needed
2.5 liters of 30% solution are needed
check answer:
(2) +%28+.7a+%2B+.3b+%29+%2F+5+=+.5+
(2) +%28+.7%2A2.5+%2B+.3%2A2.5+%29+%2F+5+=+.5+
(2) +%28+1.75+%2B+.75+%29+=+5%2A.5+
(2) +2.5+=+2.5+
OK