document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=985097>Question 985097</A>: <I><SPAN STYLE=\"word-break: break-all;\"> Against the wind a commercial airline in South America flew 840 miles in 4 hours. With a tailwind the return trip took 3.5 hours. What was the speed of the plane in the air? What w </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #605945 by </B><A HREF=/tutors/aboutme.mpl?userid=ikleyn><B>ikleyn(52794)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=ikleyn><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=ikleyn><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=605945\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> \r<BR>\n" );
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document.write( "Let &nbsp;<B>u</B>&nbsp; be the plane speed in the still air &nbsp;(in miles per hour)&nbsp; and let &nbsp;<B>v</B>&nbsp; be the wind speed &nbsp;(also in &nbsp;<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=mi%2Fh ALIGN=MIDDLE  ALT=\"mi%2Fh\"   DISABLED_style= \"max-width:90%; height:auto;\" >).\r<BR>\n" );
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document.write( "Then the speed of the plane against the wind is &nbsp;<B>u - v</B>&nbsp; (relative to the earth), &nbsp;while the speed with the tailwind is &nbsp;<B>u + v</B>.\r<BR>\n" );
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document.write( "Thus you have the system of two equation for two unknowns\r<BR>\n" );
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document.write( "<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=system%28840+=+4%2A%28u+-+v%29%2C%0D%0A840+=+3.5%2A%28u+%2B+v%29%29 ALIGN=MIDDLE  ALT=\"system%28840+=+4%2A%28u+-+v%29%2C%0D%0A840+=+3.5%2A%28u+%2B+v%29%29\"   DISABLED_style= \"max-width:90%; height:auto;\" >,\r<BR>\n" );
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document.write( "or, &nbsp;dividing the first equation by &nbsp;4&nbsp; and the second equation by &nbsp;3.5\r<BR>\n" );
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document.write( "<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=system%28u+-+v+=+210%2C%0D%0Au+%2B+v+=+240%29 ALIGN=MIDDLE  ALT=\"system%28u+-+v+=+210%2C%0D%0Au+%2B+v+=+240%29\"   DISABLED_style= \"max-width:90%; height:auto;\" >.\r<BR>\n" );
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document.write( "Add the equations in the last system. &nbsp;You will get\r<BR>\n" );
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document.write( "2u = 450.\r<BR>\n" );
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document.write( "Hence, &nbsp;<B>u</B> = <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=450%2F2 ALIGN=MIDDLE  ALT=\"450%2F2\"   DISABLED_style= \"max-width:90%; height:auto;\" > = 225 <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=mi%2Fh ALIGN=MIDDLE  ALT=\"mi%2Fh\"   DISABLED_style= \"max-width:90%; height:auto;\" >. &nbsp;It is the speed of the plane in still air.\r<BR>\n" );
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document.write( "Next, &nbsp;substitute this value of &nbsp;<B>u</B>&nbsp; into the second equation, &nbsp;and you will get &nbsp;<B>v</B> = 15 <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=mi%2Fh ALIGN=MIDDLE  ALT=\"mi%2Fh\"   DISABLED_style= \"max-width:90%; height:auto;\" >&nbsp; for the wind speed. \r<BR>\n" );
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document.write( "<B>Answer</B>. &nbsp;The plane speed is &nbsp;225 <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=mi%2Fh ALIGN=MIDDLE  ALT=\"mi%2Fh\"   DISABLED_style= \"max-width:90%; height:auto;\" >&nbsp; in the still air. &nbsp;The wind speed is &nbsp;15 <IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=mi%2Fh ALIGN=MIDDLE  ALT=\"mi%2Fh\"   DISABLED_style= \"max-width:90%; height:auto;\" >.\r<BR>\n" );
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document.write( "For more wind and current problems see my lessons in this site\r<BR>\n" );
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document.write( "&nbsp;&nbsp;&nbsp;&nbsp;- <A  rel=nofollow HREF=http://www.algebra.com/algebra/homework/word/travel/Wind-and-Current-problems.lesson>Wind and Current problems</A>, \r<BR>\n" );
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document.write( "&nbsp;&nbsp;&nbsp;&nbsp;- <A  rel=nofollow HREF=http://www.algebra.com/algebra/homework/word/travel/Wind-and-Current-problems-solvable-by-quadratic-equations.lesson>Wind and Current problems solvable by quadratic equations</A>.\r<BR>\n" );
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