document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=98403>Question 98403</A>: <I><SPAN STYLE=\"word-break: break-all;\"> The credit remaining on a phone card (in dollars) is a linear function of the total calling time made with the card (in minutes), as shown in the figure below. The remaining credit after 23 minutes of calls is $21.78 , and the remaining credit after 57 minutes of calls is $17.02 . What is the remaining credit after 87 minutes of calls? <BR>\n" );
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document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #71568 by </B><A HREF=/tutors/aboutme.mpl?userid=mathslover><B>mathslover(157)</B></A><img src=\"/images/stars/stars-09.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=mathslover><IMG SRC=http://www.algebra.com/images/aboutme-small.gif BORDER=0 alt=\"About Me\" VALIGN=MIDDLE></A>&nbsp;<IMG SRC=http://www.algebra.com/cgi-bin/show-face.mpl?user=mathslover><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=71568\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> let the total credit C after  M minutes of call be given by <BR>\n" );
document.write( "C = $(a*M + b) where a and b are some constants.\r<BR>\n" );
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document.write( "So we have: <BR>\n" );
document.write( "21.78 = a * 23 + b       (1)    and<BR>\n" );
document.write( "17.02 = a * 57 + b       (2)\r<BR>\n" );
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document.write( "Subtracting equation (2) from (1):\r<BR>\n" );
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document.write( "4.76 = -34a<BR>\n" );
document.write( "a =  -4.76/34<BR>\n" );
document.write( "  =  -.14\r<BR>\n" );
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document.write( "Putting this value of a in equation (1):<BR>\n" );
document.write( "21.78 = -.14 * 23 + b\r<BR>\n" );
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document.write( "therefore, b = 21.78 + 3.22<BR>\n" );
document.write( "             = 25\r<BR>\n" );
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document.write( "So the linear equation for the credit remaining after M minutes of call is <BR>\n" );
document.write( "C = $(-.14 * M + 25)\r<BR>\n" );
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document.write( "So, the remaining credit after 87 minutes<BR>\n" );
document.write( "C = -.14 * 87 + 25 <BR>\n" );
document.write( "  = $12.82      </SPAN>\n" );
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