document.write( "Question 729969: The speed of the current in a river is 5 miles per hour. A boat leaves a dock on the bank of the river, travels upstream 25 miles, and returns to the dock in 12 hours. What is the speed of the boat in still water? \n" ); document.write( "

Algebra.Com's Answer #482331 by Finavon(81) $\"\"$ $\"About$

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$\"+Time+=+Distance%2FSpeed+\"$
\n" ); document.write( "Boat speed: x mph
\n" ); document.write( "Total time = $\"25%2F%28x-5%29+%2B+25%2F%28x%2B5%29+=+12+\"$
\n" ); document.write( "Multiply by (x-5)*(x+5)
\n" ); document.write( "25*(x+5) + 25*(x-5) = 12*(x-5)*(x+5)
\n" ); document.write( " $\"+50x+=+12x%5E2+-+12%2A25+\"$
\n" ); document.write( " $\"+12x%5E2+-+50x+-+300+=+0+\"$
\n" ); document.write( " $\"+6x%5E2+-+25x+-+150+=+0+\"$
\n" ); document.write( "Solves for $\"+x+=+%2825+%2B-sqrt%2825%5E2+-4%2A6%2A%28-150%29%29%29%2F%282%2A6%29+\"$
\n" ); document.write( " $\"+x+=+%2825+%2B-sqrt%28625%2B3600%29%29%2F12+\"$
\n" ); document.write( "Real x = 90/12 = 7.5 mph
\n" ); document.write( "Check:
\n" ); document.write( "25 miles @ 2.5 mph 10 hrs
\n" ); document.write( "25 miles @ 12.5 mph 2 hours
\n" ); document.write( "Total 12 hrs
\n" ); document.write( " \n" ); document.write( "

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