document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=840724>Question 840724</A>: <I><SPAN STYLE=\"word-break: break-all;\"> Okay this is the problem and i need to use elimination or substitution to solve. I have no idea how to get the equations.\r<BR>\n" );
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document.write( "A motorboat can go 12 miles downstream on a river in 20 minutes. It takes 30 minutes for this boat to go back upstream the same 12 miles. Find the speed of the current.<BR>\n" );
document.write( "(btw it won't let me choose a topic) </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #506426 by </B><A HREF=/tutors/aboutme.mpl?userid=ankor@dixie-net.com><B>ankor@dixie-net.com(22740)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=ankor@dixie-net.com><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=ankor@dixie-net.com><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=506426\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> A motorboat can go 12 miles downstream on a river in 20 minutes.<BR>\n" );
document.write( " It takes 30 minutes for this boat to go back upstream the same 12 miles.<BR>\n" );
document.write( " Find the speed of the current.<BR>\n" );
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document.write( "The idea is to understand what's going on here<BR>\n" );
document.write( "We probably want the speeds in mph, therefore<BR>\n" );
document.write( "change 20 min to 1/3 of an hr<BR>\n" );
document.write( "change 30 min to 1/2 hr<BR>\n" );
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document.write( "let s = the speed of the boat in still water<BR>\n" );
document.write( "let c = rate of the current<BR>\n" );
document.write( "then<BR>\n" );
document.write( "(s+c) = effective speed down stream<BR>\n" );
document.write( "and<BR>\n" );
document.write( "(s-c) = effective speed upstream<BR>\n" );
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document.write( "Write a distance equation for each way; dist = speed * time<BR>\n" );
document.write( "<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=1%2F3 ALIGN=MIDDLE  ALT=\"1%2F3\"   DISABLED_style= \"max-width:90%; height:auto;\" >(s+c) = 12; downstream<BR>\n" );
document.write( "<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=1%2F2 ALIGN=MIDDLE  ALT=\"1%2F2\"   DISABLED_style= \"max-width:90%; height:auto;\" >(s-c) = 12; upstream<BR>\n" );
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document.write( "Simplify this, multiply the 1st equation by 3, the 2nd equation by 2<BR>\n" );
document.write( "s + c = 36<BR>\n" );
document.write( "s - c = 24<BR>\n" );
document.write( "---------------Adding eliminates c, find s<BR>\n" );
document.write( "2s = 60<BR>\n" );
document.write( "s = 60/2<BR>\n" );
document.write( "s = 30 mph in still water<BR>\n" );
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document.write( "But they want the speed of the current<BR>\n" );
document.write( "s + c = 36<BR>\n" );
document.write( "replace s with 30<BR>\n" );
document.write( "30 + c = 36<BR>\n" );
document.write( "c = 36 - 30<BR>\n" );
document.write( "c = 6 mph is the current<BR>\n" );
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document.write( "Check this in the 1st original equation<BR>\n" );
document.write( "<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=1%2F3 ALIGN=MIDDLE  ALT=\"1%2F3\"   DISABLED_style= \"max-width:90%; height:auto;\" >(30+6) =  <BR>\n" );
document.write( "<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=1%2F3 ALIGN=MIDDLE  ALT=\"1%2F3\"   DISABLED_style= \"max-width:90%; height:auto;\" >* 36 = 12<BR>\n" );
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document.write( "You can confirm this in the 2nd original equation<BR>\n" );
document.write( "<IMG STYLE=\"max-width:80%;\" SRC=/cgi-bin/plot-formula.mpl?expression=1%2F2 ALIGN=MIDDLE  ALT=\"1%2F2\"   DISABLED_style= \"max-width:90%; height:auto;\" >(s-c) = <BR>\n" );
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document.write( "Is this starting to make sense to you now?<BR>\n" );
document.write( " </SPAN>\n" );
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