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Let x = no. of .15 stamps; y = no. of .20 stamps; z = no. of .40 stamps
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\n" ); document.write( "Write two equations from this information:
\n" ); document.write( "An account executive bought 330 stamps for $79.50.
\n" ); document.write( "1st eq: x + y + z = 330
\n" ); document.write( "and
\n" ); document.write( "2nd eq: .15x + .20y + .40z = 79.50
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\n" ); document.write( " The number of $.20 stamps is four times the number of $.15 stamps.
\n" ); document.write( " y = 4x
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\n" ); document.write( "Substitute 4x for y in the 1st equation:
\n" ); document.write( "x + 4x + z = 330
\n" ); document.write( "5x + z = 330
\n" ); document.write( "z = (330 - 5x);
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\n" ); document.write( "In the 2nd equation, substitute 4x for y and (330-5x) for z; find x:
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\n" ); document.write( ".15x + .20(4x) + .40(330-5x) = 79.50
\n" ); document.write( ".15x + .80x + 132 - 2x = 79.50
\n" ); document.write( ".15x + .80x - 2x = 79.50 - 132
\n" ); document.write( "-1.05x = -52.5
\n" ); document.write( "x = -52.5/-1.05
\n" ); document.write( "x = 50 ea .15 stamps
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\n" ); document.write( "y = 4x
\n" ); document.write( "y = 4(50)
\n" ); document.write( "y = 200 ea .20 stamps
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\n" ); document.write( " How many $.40 stamps were purchased?
\n" ); document.write( "Use the total number of stamps purchased:
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\n" ); document.write( "330 - 50 - 200 = 80 ea .40 stamps
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\n" ); document.write( "Check solutions using the 2nd equation:
\n" ); document.write( ".15(50) + .20(200) + .40(80) = 79.50
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\n" ); document.write( "Did this make sense to you? \n" ); document.write( "