SOLUTION: A metallurgist has one alloy containing 34% copper and another containing 55% copper. How many pounds of each alloy must he use to make 49 pounds of a third alloy containing 40% co

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Question 758245: A metallurgist has one alloy containing 34% copper and another containing 55% copper. How many pounds of each alloy must he use to make 49 pounds of a third alloy containing 40% copper?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +a+ = pounds of 34% alloy needed
Let +b+ = pounds of 55% alloy needed
+.34a+ = pounds of copper in +a+
+.55b+ = pounds of copper in +b+
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given:
(1) +a+%2B+b+=+49+
(2) +%28+.34a+%2B+.55b+%29+%2F+49+=+.4+
----------------------------
(2) +.34a+%2B+.55b+=+.4%2A49+
(2) +.34a+%2B+.55b+=+19.6+
(2) +34a+%2B+55b+=+1960+
Multiply both sides of (1) by +34+ and
subtract (1) from (2)
(2) +34a+%2B+55b+=+1960+
(1) +-34a+-+34b+=+-1666+
+21b+=+294+
+b+=+14+
and, since
(1) +a+%2B+b+=+49+
(1) +a+%2B+14+=+49+
(1) +a+=+35+
35 pounds of 34% alloy are needed
14 pounds of 55% alloy are needed
check:
(2) +%28+.34a+%2B+.55b+%29+%2F+49+=+.4+
(2) +%28+.34%2A35+%2B+.55%2A14+%29+%2F+49+=+.4+
(2) ++11.9+%2B+7.7+=+.4%2A49+
(2) +19.6+=+19.6+
OK