document.write( "Question 196396: mary travels 45 miles up stream in 3 hours and travels 100 miles downstream in 4 hours. find the average speed of the boat in still water and the average speed of the current? \n" ); document.write( "
Algebra.Com's Answer #147238 by nerdybill(7384)\"\" \"About 
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mary travels 45 miles up stream in 3 hours and travels 100 miles downstream in 4 hours. find the average speed of the boat in still water and the average speed of the current?
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\n" ); document.write( "Applying the distance formula:
\n" ); document.write( "d = rt
\n" ); document.write( "where
\n" ); document.write( "d is distance
\n" ); document.write( "r is rate or speed
\n" ); document.write( "t is time
\n" ); document.write( ".
\n" ); document.write( "Let x = speed of boat in still water
\n" ); document.write( "and y = speed of current
\n" ); document.write( ".
\n" ); document.write( "Upstream:
\n" ); document.write( "3(x-y)= 45
\n" ); document.write( "dividing both sides by 3:
\n" ); document.write( "x - y = 15
\n" ); document.write( ".
\n" ); document.write( "Downstream:
\n" ); document.write( "4(x+y)= 100
\n" ); document.write( "dividing both sides by 4:
\n" ); document.write( "x + y = 25
\n" ); document.write( ".
\n" ); document.write( "Our system of equations:
\n" ); document.write( "x - y = 15 (equation 1)
\n" ); document.write( "x + y = 25 (equation 2)
\n" ); document.write( ".
\n" ); document.write( "Adding the two equations:
\n" ); document.write( "x - y = 15
\n" ); document.write( "x + y = 25
\n" ); document.write( "-----------
\n" ); document.write( "2x = 40
\n" ); document.write( "x = 20 mph (speed of boat in still waters)
\n" ); document.write( ".
\n" ); document.write( "substitute the above into equation 2 and solve for y:
\n" ); document.write( "x + y = 25
\n" ); document.write( "20 + y = 25
\n" ); document.write( "y = 5 mph (speed of current) \n" ); document.write( "
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