SOLUTION: A chemist is mixing a 40% salt solution with a 20% salt solution to make 50 L of a new solution that will contain 25% salt. How much of the 40% solution should the chemist use?

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Question 648118: A chemist is mixing a 40% salt solution with a 20% salt solution to make 50 L of a new solution that will contain 25% salt. How much of the 40% solution should the chemist use?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +a+ = liters of 40% solution needed
Let +b+ = liters of 20% solution needed
+.4a+ = liters of salt in 40% solution
+.2b+ = liters of salt in 20% solution
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(1) +a+%2B+b+=+50+
(2) +%28+.4a+%2B+.2b+%29+%2F+50+=+.25+
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(2) +.4a+%2B+.2b+=+.25%2A50+
(2) +.4a+%2B+.2b+=+12.5+
(2) +4a+%2B+2b+=+125+
---------------------
Multiply both sides of (1) by 2
and subtract (1) from (2)
(2) +4a+%2B+2b+=+125+
(1) +-2a+-+2b+=+-100+
+2a+=+25+
+a+=+12.5+
and, since
(1) +a+%2B+b+=+50+
(1) +12.5+%2B+b+=+50+
(1) +b+=+37.5+
12.5 liters of the 40% solution are needed
check:
(2) +%28+.4a+%2B+.2b+%29+%2F+50+=+.25+
(2) +%28+.4%2A12.5+%2B+.2%2A37.5+%29+%2F+50+=+.25+
(2) +%28+5+%2B+7.5+%29+%2F+50+=+.25+
(2) +12.5+=+.25%2A50+
(2) +12.5+=+12.5+
OK