SOLUTION: one alloy is 50% silver and another is 20% silver. How much of each alloy is needed to make 100 kilograms of alloy which is 45% silver?

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Question 845168: one alloy is 50% silver and another is 20% silver. How much of each alloy is needed to make 100 kilograms of alloy which is 45% silver?
Found 2 solutions by Alan3354, josmiceli:
Answer by Alan3354(69443) About Me  (Show Source):
You can put this solution on YOUR website!

Do it like this one. Only the numbers are different.
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Amanda wants to make 8 gal. of a 20% saline solution by mixing together a 56% saline solution and a 8% saline solution. How much of each solution must she use?
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e = amount of 8%
f = amount of 56%
---
e+ f = 8 (total solution)
8e + 56f = 20*8 (total saline)
---
e+ f = 8
e + 7f = 20
------------ Subtract
-6f = -12
f = 2
e = 6

Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +a+ = kg of 50% silver needed
Let +b+ = kg of 20% silver needed
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(1) +a+%2B+b+=+100+
(2) +%28+.5a+%2B+.2b+%29+%2F+100+=+.45+
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(2) +.5a+%2B+.2b+=+45+
(2) +5a+%2B+2b+=+450+
Multiply both sides of (1) by +2+
and subtract (1) from (2)
(2) +5a+%2B+2b+=+450+
(1) +-2a+-+2b+=+-200+
+3a+=+250+
+a+=+83.333+
and
(1) +a+%2B+b+=+100+
(1) +b+=+100+-+83.333+
(1) +b+=+16.667+
83.333 kg of 50% silver is needed
16.667 kg of 20% silver is needed
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check:
(2) +%28+.5a+%2B+.2b+%29+%2F+100+=+.45+
(2) +%28+.5%2A83.333+%2B+.2%2A16.667+%29+%2F+100+=+.45+
(2) +%28+41.6665+%2B+3.3334+%29+%2F+100+=+.45+
(2) +45.00005+=+45+
close enough