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Pencils cost 25 cents apiece. Pens cost 80 cents apiece. Susan spent $20 for a total of 36 pens and pencils. How many pencils did she buy?
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document.write( "Let P = number of pencils.
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document.write( "Let E = number of pens.\r
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document.write( "Susan spent $20 dollars on 36 pens and pencils so we know:
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document.write( "We also know that Susan bought 36 things so we also know:
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document.write( "Substitute P with 36-E in the first equation:\r
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document.write( "So, .
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document.write( "Or, .
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document.write( "So, Susan bought 20 pens. She must have then bought 16 pencils since she bought 36 things altogether.\r
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document.write( "We verify these numbers by noting that 20 pens cost 20*80 = $16 and 16 pencils cost 16*.25 = $4 and the total is $20.