document.write( "Question 1131057: the speed of a boat in still water is 15 km/hr. it needs four more hours to travel 63 km against the current of a river than it needs to travel down the river. Determine the speed
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Algebra.Com's Answer #747680 by ankor@dixie-net.com(22740)\"\" \"About 
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the speed of a boat in still water is 15 km/hr.
\n" ); document.write( " it needs four more hours to travel 63 km against the current of a river than it needs to travel down the river.
\n" ); document.write( " Determine the speed of the current
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\n" ); document.write( "Let c = the rate of the current
\n" ); document.write( "then
\n" ); document.write( "(15-c) = actual speed against the current
\n" ); document.write( "and
\n" ); document.write( "(15+c) = actual speed with the current
\n" ); document.write( ":\r
\n" ); document.write( "\n" ); document.write( "Write a time equation: time = dist/speed
\n" ); document.write( "Upstream time - downstream time = 4 hrs
\n" ); document.write( "\"63%2F%28%2815-c%29%29\" - \"63%2F%28%2815%2Bc%29%29\" = 4
\n" ); document.write( "multiply equation by (15-c)(15+c), cancel the denominators
\n" ); document.write( "63(15+c) - 63(15-c) = 4(15-c)(15+c)
\n" ); document.write( "945 + 63c - 945 + 63c = 4(225-c^2)
\n" ); document.write( "126c = 900 - 4c^2
\n" ); document.write( "form a quadratic equation on the left
\n" ); document.write( "4c^2 + 126c - 900 = 0
\n" ); document.write( "Use the quadratic formula a=4; b=126; =-900
\n" ); document.write( "I got solutions of
\n" ); document.write( "c = -37.5, not reasonable
\n" ); document.write( "and
\n" ); document.write( "c = 6 km/hr is the speed of the current
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\n" ); document.write( "See if that works, find the actual time each way, (9 km/hr up & 21 km/hr down)
\n" ); document.write( "63/9 = 7 hrs
\n" ); document.write( "63/21= 3 hrs
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\n" ); document.write( "time dif: 4 hrs
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