document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=375325>Question 375325</A>: <I><SPAN STYLE=\"word-break: break-all;\"> A boat travels 80 miles upstream in a river with a current of 6 miles per hour; then it travels back down to its starting point.  The total time of the trip was 4 hours.  Find the speed of the boat in still water.  what is the equation too? </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #266875 by </B><A HREF=/tutors/aboutme.mpl?userid=Jk22><B>Jk22(389)</B></A><img src=\"/images/stars/stars-10.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=Jk22><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=Jk22><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=266875\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> let <BR>\n" );
document.write( "v>0 speed of the boat in still water, tup, time upstream<BR>\n" );
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document.write( "upstream travel : 80 = (v-6)*tup <BR>\n" );
document.write( "downstream      : 80 = (v+6)*tdown<BR>\n" );
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document.write( "tup+tdown = 80/(v-6)+80/(v+6) = 4<BR>\n" );
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document.write( "80(v+6)+80(v-6) = 4(v^2-36)<BR>\n" );
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document.write( "40v = v^2 - 36 => v^2 - 40v - 36 = 0 = (v - 20)^2 - 400 - 36<BR>\n" );
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document.write( "(v-20)^2 = 436 = 218*2 = 109*4<BR>\n" );
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document.write( "v = 20 + 2Sqrt(109) mph<BR>\n" );
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