document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=500351>Question 500351</A>: <I><SPAN STYLE=\"word-break: break-all;\"> I need to know the formula to get this answer. \r<BR>\n" );
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document.write( "Mike drove uphill to his school and it took 3 hours. He returned downhill only going 5mph faster and made it in two hours. What is the average rate of speed? </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #338125 by </B><A HREF=/tutors/aboutme.mpl?userid=Alan3354><B>Alan3354(69443)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=Alan3354><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=Alan3354><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=338125\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> I need to know the formula to get this answer. <BR>\n" );
document.write( "d = r*t is the formula<BR>\n" );
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document.write( "Mike drove uphill to his school and it took 3 hours. He returned downhill only going 5mph faster and made it in two hours. What is the average rate of speed?<BR>\n" );
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document.write( "d = r*t<BR>\n" );
document.write( "d = r*3<BR>\n" );
document.write( "d = (r+5)*2<BR>\n" );
document.write( "3r = 2(r+5)<BR>\n" );
document.write( "r1 = 10 mi/hr uphill<BR>\n" );
document.write( "r2 = 15 mi/hr downhill<BR>\n" );
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document.write( "Here's a formula:<BR>\n" );
document.write( "avg speed = 2*r1*r2/(r1 + r2)<BR>\n" );
document.write( "= 2*10*15/25 = 300/25<BR>\n" );
document.write( "= 12 mi/hr<BR>\n" );
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document.write( "Notice that the distance is irrelevant, but they have to be equal for the formula to apply.<BR>\n" );
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document.write( "A different method, without the formula:<BR>\n" );
document.write( "d = 10*3 = 30 miles<BR>\n" );
document.write( "round trip = 60 miles<BR>\n" );
document.write( "Total time = 3 hr + 2 hr = 5 hours<BR>\n" );
document.write( "Avg speed = 60/5 = 12 mi/hr  same answer<BR>\n" );
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