SOLUTION: A pharmacist needs 70 liters of a 50% alcohol solution. She has availble a 30% alcohole solution and an 80% alocohol solution. How many liters of each solution should she mix to ob

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Question 677693: A pharmacist needs 70 liters of a 50% alcohol solution. She has availble a 30% alcohole solution and an 80% alocohol solution. How many liters of each solution should she mix to obtain 70 liters of wach solution should she mix to obtain 70 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 30% solution needed
Let +b+ = liters of 80% solution needed
given:
(1) +a+%2B+b+=+70+
(2) +%28+.3a+%2B+.8b+%29+%2F+70+=+.5+
-------------------------
(2) +.3a+%2B+.8b+=+.5%2A70+
(2) +.3a+%2B+.8b+=+35+
(2) +3a+%2B+8b+=+350+
Multiply both sides of (1) by +3+ and
subtract (1) from (2)
(2) +3a+%2B+8b+=+350+
(1) +-3a+-+3b+=+-210+
+5b+=+140+
+b+=+28+
and, since
(1) +a+%2B+b+=+70+
(1) +a+%2B+28+=+70+
(1) +a+=+42+
42 liters of 30% solution are needed
28 liters of 80% solution are needed
check answer:
(2) +%28+.3%2A42+%2B+.8%2A28+%29+%2F+70+=+.5+
(2) +%28+12.6+%2B+22.4+%29+%2F+70+=+.5+
(2) +35+=+.5%2A70+
(2) +35+=+35+
OK