document.write( "Question 183868: Joanna took 30 min. to drive her boat upstream to water-ski at her favorite spot. Coming back later that day, at the same boat speed, took her 25 min. If the current in that part of the river is 5 km per hr. What is the average speed of the boat? \n" ); document.write( "
Algebra.Com's Answer #138006 by ptaylor(2198)\"\" \"About 
You can put this solution on YOUR website!
Distance(d) equals Rate(r) times Time(t) or d=rt; r=d/t and t=d/r\r
\n" ); document.write( "\n" ); document.write( "Average speed =(Total Distance)/(Total Time)\r
\n" ); document.write( "\n" ); document.write( "Let r=speed of the boat in still water
\n" ); document.write( "Distance upstream=(r-5)*(1/2) {30 min=1/2 hr}
\n" ); document.write( "Distance downstream=(r+5)*(5/12)\r
\n" ); document.write( "\n" ); document.write( "Distance Upstream = Distance Downstream, so:
\n" ); document.write( "(r-5)(1/2)=(r+5)(5/12) multiply each term by 12
\n" ); document.write( "6(r-5)=5(r+5) get rid of parens
\n" ); document.write( "6r-30=5r+25 subtract 5r from and add 30 to each side
\n" ); document.write( "6r-5r-30+30=5r-5r+25+30 collect like terms
\n" ); document.write( "r=55 mph---------------------speed of the boat in still water
\n" ); document.write( "Distance Upstream =(50-5)*(1/2)=25 mi
\n" ); document.write( "Distance Downstream=(55+5)*(5/12)=25 mi
\n" ); document.write( "Total distance=50 mi
\n" ); document.write( "Total Time=(5/12)+(6/12)=11/12 hr
\n" ); document.write( "Average Speed of Boat =50/(11/12)=54.5 mph\r
\n" ); document.write( "\n" ); document.write( "Hope this helps---ptaylor
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