document.write( "Question 442293: A boat travels 2 km upstream and 2km back. The time for the round trip is 4hrs. The speed of the stream is 4km/hr. What is the speed of the boat in still water? \n" ); document.write( "
Algebra.Com's Answer #305033 by stanbon(75887)\"\" \"About 
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A boat travels 2 km upstream and 2km back. The time for the round trip is 4hrs. The speed of the stream is 4km/hr. What is the speed of the boat in still water?
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\n" ); document.write( "Upstream DATA:
\n" ); document.write( "distance = 2 km; rate = b-4 km ; time = 2/(b-4)hrs
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\n" ); document.write( "Downstream DATA:
\n" ); document.write( "distance = 2 km ; rate = b+4 km ; time = 2/(b+4) hrs
\n" ); document.write( "-------
\n" ); document.write( "Equations:
\n" ); document.write( "time + time = 4 hrs
\n" ); document.write( "2/(b-4) + 2/(b+4) = 4
\n" ); document.write( "---
\n" ); document.write( "Multiply thru by (b-4)(b+4)
\n" ); document.write( "2(b+4) + 2(b-4) = 4b^2-64
\n" ); document.write( "4b = 4b^2-64
\n" ); document.write( "b^2-b-16 = 0
\n" ); document.write( "---
\n" ); document.write( "b = [1 +- sqrt(1-4*1*-16)]/2
\n" ); document.write( "----
\n" ); document.write( "b = [1 +- sqrt(65)]/2
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\n" ); document.write( "Positive solution:
\n" ); document.write( "b = [1+sqrt(65)]/2 = 4.53 km/hr (speed in still water)
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\n" ); document.write( "Cheers,
\n" ); document.write( "Stan H. \n" ); document.write( "
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