SOLUTION: By weight,one fertilizer is 10% potassium, 40% nitrogen and 50% phosphorous. A second fertilizer has percents 10, 20 & 70, respectively and a third fertilizer has percents of 0, 40

Algebra ->  Customizable Word Problem Solvers  -> Mixtures -> SOLUTION: By weight,one fertilizer is 10% potassium, 40% nitrogen and 50% phosphorous. A second fertilizer has percents 10, 20 & 70, respectively and a third fertilizer has percents of 0, 40      Log On

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Question 630935: By weight,one fertilizer is 10% potassium, 40% nitrogen and 50% phosphorous. A second fertilizer has percents 10, 20 & 70, respectively and a third fertilizer has percents of 0, 40, and 60 respectively. How much of each must be mixed to get 200 lbs of fertilizer with percents of 8, 30 & 62, respectively?
___ lbs of first mixture
___ lbs of second mixture
___ lbs of third mixture

Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
+.08%2A200+=+16+ pounds of potassium total
+.3%2A200+=+60+ pounds of nitrogen total
+.62%2A200+=+124+ pounds of phosphorus total
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Let +a+ = weight in pounds of 1st fertilizer
Let +b+ = weight in pounds of 2nd fertilizer
Let +c+ = weight in pounds of 3rd fertilizer
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+.1a+%2B+.1b+%2B+0%2Ac+=+16+
+.4a+%2B+.2b+%2B+.4c+=+60+
+.5a+%2B+.7b+%2B+.6c+=+124+
----------------------
(1) +a+%2B+b+=+160+
(2) +4a+%2B+2b+%2B+4c+=+600+
(3) +5a+%2B+7b+%2B+6c+=+1240+
------------------------
(1) +a+=+160+-+b+
Substitute this into (2)
(2) +4%2A%28+160+-+b+%29+%2B+2b+%2B+4c+=+600+
(2) +640+-+4b+%2B+2b+%2B+4c+=+600+
(2) +-2b+%2B+4c+=+600+-+640+
(2) +4c+=+2b+-+40+
(2) +c+=+b%2F2+-+10+
Substitute (1) and (2) into (3)
(3) +5%2A%28+160+-+b+%29+%2B+7b+%2B+6%2A%28+b%2F2+-+10+%29+=+1240+
(3) +800+-+5b+%2B+7b+%2B+3b+-+60+=+1240+
(3) +5b+=+1240+-+800+%2B+60+
(3) +5b+=+500+
(3) +b+=+100+
and
(1) +a+=+160+-+b+
(1) +a+=+160+-+100+
(1) +a+=+60+
and
(2) +c+=+b%2F2+-+10+
(2) +c+=+100%2F2+-+10+
(2) +c+=+40+
60 pounds of 1st fertilizer are needed
100 pounds of 2nd fertilizer are needed
40 pounds of 3rd fertilizer are needed
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You can check like this:
+.1a+%2B+.1b+%2B+0%2Ac+=+16+
+.1%2A60+%2B+.1%2A100+=+16+
+6+%2B+10+=+16+
+16++=+16+
OK- you can check (2) and (3)