document.write( "Question 80784:
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Algebra.Com's Answer #57935 by ankor@dixie-net.com(22740)\"\" \"About 
You can put this solution on YOUR website!
The Hudson River flows at a rate of 3 miles per hour. A patrol boat travels 60 miles upriver, and returns in a total time of 9 hours. What is the speed of the boat in still water?
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\n" ); document.write( "Let s = speed of the boat in still water
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\n" ); document.write( "(s-3) = speed up-river
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\n" ); document.write( "(s+3) = speed down-river
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\n" ); document.write( "Write a time equation: Time = distance/speed
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\n" ); document.write( "Time up-river + Time down-river = 9 hrs
\n" ); document.write( "\"60%2F%28s-3%29\" + \"60%2F%28s%2B3%29\" = 9
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\n" ); document.write( "Multiply equation by (s+3)(s-3) and you have:
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\n" ); document.write( "60(s+3) + 60(s-3) = 9(s+3)(s-3)
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\n" ); document.write( "60s + 180 + 60s - 180 = 9(s^2 - 9)
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\n" ); document.write( "120s = 9s^2 - 81
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\n" ); document.write( "9s^2 - 120s - 81 = 0; a quadratic equation
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\n" ); document.write( "Use the quadratic equation to find s: a=9; b=-120; c=-81
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\n" ); document.write( "\"s+=+%28-%28-120%29+%2B-+sqrt%28-120%5E2+-+4%2A9%2A-81+%29%29%2F%282%2A9%29+\"
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\n" ); document.write( "\"s+=+%28%2B120+%2B-+sqrt%2814400+%2B+2916+%29%29%2F%2818%29+\"
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\n" ); document.write( "Do the math, you should get a positive solution of:
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\n" ); document.write( "s = 13.977 mph, speed in still water
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\n" ); document.write( "Check on calc, find the times at each speed:
\n" ); document.write( "60/16.977 + 60/10.977 = 9.00 hrs\r
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