document.write( "Question 190306: A cruise boat travels 60 miles downstream in 3 hours and returns to its starting point upstream in 12 hours. Find the speed of the stream. \n" ); document.write( "
Algebra.Com's Answer #142828 by orca(409)\"\" \"About 
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Let x represent the speed of the boat in still water.
\n" ); document.write( "Let y represent the speed of the stream.\r
\n" ); document.write( "\n" ); document.write( "Then
\n" ); document.write( "the speed of the boat downstream = x + y
\n" ); document.write( "the speed of the boat upstream = x - y\r
\n" ); document.write( "\n" ); document.write( "Since the boat travels 60 miles downstream in 3 hours, we can write the first equation as:
\n" ); document.write( "3(x+y) = 60 .....................(1)\r
\n" ); document.write( "\n" ); document.write( "Since the boat returns to its starting point upstream in 12 hours, we can write the second equation as:
\n" ); document.write( "12(x-y) = 60 .....................(2)\r
\n" ); document.write( "\n" ); document.write( "After simplifying , equations (1) and (2) becomes:
\n" ); document.write( "x + y = 20 ................(3)
\n" ); document.write( "x - y = 5 ................(4)\r
\n" ); document.write( "\n" ); document.write( "Adding up (3) and (4), we have
\n" ); document.write( "x + y + x - y = 20 + 5
\n" ); document.write( "2x = 25
\n" ); document.write( "x = 12.5\r
\n" ); document.write( "\n" ); document.write( "Substitute x = 12.5 into equation (3), we have
\n" ); document.write( "12.5 + y = 20
\n" ); document.write( "Solving y, we have
\n" ); document.write( "y = 20 - 12.5
\n" ); document.write( "y = 7.5
\n" ); document.write( "So the speed of the stream is 7.5 miles per hour.
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