document.write( "Question 177648: Joann took 20 miuntes to drive her boat upstream to water-ski at her favorite spot. Coming back later in the day, at the same boat speed, took her 15 minutes. If the current in that part of the river is 5 km per hr, what was her boat speed? \n" ); document.write( "
Algebra.Com's Answer #132711 by gonzo(654)\"\" \"About 
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equation you are looking for is rate * time = distance.
\n" ); document.write( "going upstream she is working against the current (combined rate = boat minus current)
\n" ); document.write( "coming downstream she is working with the current (combined rate = boat plus current).
\n" ); document.write( "let x = the rate of the boat.
\n" ); document.write( "let d = the distance.
\n" ); document.write( "going upstream:
\n" ); document.write( "(x-5)*20 = d
\n" ); document.write( "coming downstream:
\n" ); document.write( "(x+5)*15 = d
\n" ); document.write( "since both equations equal d, then they are equal to each other, so:
\n" ); document.write( "(x-5)*20 = (x+5)*15
\n" ); document.write( "expanding, this becomes:
\n" ); document.write( "20*x - 100 = 15*x + 75
\n" ); document.write( "subtract 15*x from both sides and add 100 to both sides to get:
\n" ); document.write( "20*x - 15*x = 75 + 100
\n" ); document.write( "which becomes:
\n" ); document.write( "5*x = 175
\n" ); document.write( "divide both sides by 5 to get:
\n" ); document.write( "x = 35 miles per hour
\n" ); document.write( "this means that the boat's speed is 35 miles per hour.
\n" ); document.write( "---
\n" ); document.write( "to prove this is correct, substitute in the original equation of:
\n" ); document.write( "(x-5)*20 = (x+5)*15
\n" ); document.write( "which becomes:
\n" ); document.write( "(35-5)*20 = (35+5)*15
\n" ); document.write( "which becomes:
\n" ); document.write( "30*20 = 40*15
\n" ); document.write( "divide both sides by 15 to get:
\n" ); document.write( "30/15*20 = 40
\n" ); document.write( "which becomes:
\n" ); document.write( "2*20 = 40
\n" ); document.write( "which is true so the values for the boat speed are good.
\n" ); document.write( "---
\n" ); document.write( "your answer is:
\n" ); document.write( "speed of the boat is 35 miles per hour.
\n" ); document.write( " \n" ); document.write( "
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