document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=455372>Question 455372</A>: <I><SPAN STYLE=\"word-break: break-all;\"> The question is: A river has a current of 3mph. A boat goes 35 miles upstream in the same time as it goes 45 miles downstream. Find the speed of the boat in still water?<BR>\n" );
document.write( " I tried the following: d=rt  let upstream = u and downstream =d so <BR>\n" );
document.write( "45=3u+d - (35=3u+d) Please help i don't believe I'm doing this correctly.<BR>\n" );
document.write( " </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #312648 by </B><A HREF=/tutors/aboutme.mpl?userid=oberobic><B>oberobic(2304)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=oberobic><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/images/nopicture.jpg><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=312648\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> Going upstream:<BR>\n" );
document.write( "d = rt<BR>\n" );
document.write( "r = speed of boat in still water - 3 mph current<BR>\n" );
document.write( "d = 35 (given)<BR>\n" );
document.write( "35 = (s-3)*t<BR>\n" );
document.write( "Going downstream:<BR>\n" );
document.write( "d = rt<BR>\n" );
document.write( "r = speed of boat in still water + 3 mph current<BR>\n" );
document.write( "d = 45 (given)<BR>\n" );
document.write( "45 = (s+3)*t<BR>\n" );
document.write( "We also are told t is the same for both trips.<BR>\n" );
document.write( "But we are not told what t is.<BR>\n" );
document.write( "So, we can turn the equations around to be in the form:  t =<BR>\n" );
document.write( "Then since t = t, we can solve.<BR>\n" );
document.write( ".<BR>\n" );
document.write( "35/(s-3) = t<BR>\n" );
document.write( "45/(s+3) = t<BR>\n" );
document.write( "So,<BR>\n" );
document.write( "35/(s-3) = 45/(s+3)<BR>\n" );
document.write( "Cross multiply<BR>\n" );
document.write( "35*(s+3) = 45*(s-3)<BR>\n" );
document.write( "35s + 105 = 45s - 135<BR>\n" );
document.write( "Subtract 35s from both sides<BR>\n" );
document.write( "105 = 10s -135<BR>\n" );
document.write( "Add 135 to both sides<BR>\n" );
document.write( "240 = 10s<BR>\n" );
document.write( "10s = 240<BR>\n" );
document.write( "Divide both sides by 10<BR>\n" );
document.write( "s = 24<BR>\n" );
document.write( ".<BR>\n" );
document.write( "Always check your answers!<BR>\n" );
document.write( "In this case, if you plug s=24 into the equations, will the t's be equal?<BR>\n" );
document.write( "35/(24-3) = 35/21 = 5/3<BR>\n" );
document.write( "45/(24+3) = 45/27 = 5/3<BR>\n" );
document.write( "Yes!<BR>\n" );
document.write( ".<BR>\n" );
document.write( "So the speed of the boat in still water is 24 mph.<BR>\n" );
document.write( ".<BR>\n" );
document.write( "Done. </SPAN>\n" );
document.write( "</TD></TR></TABLE>\n" );