SOLUTION: the speed of a stream is 5mph. if a boat travles 40 miles downstream in the same time it takes to travel 20 miles upstream, what is the speed of the boat in still water?

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Question 251926: the speed of a stream is 5mph. if a boat travles 40 miles downstream in the same time it takes to travel 20 miles upstream, what is the speed of the boat in still water?
Answer by checkley77(12844) About Me  (Show Source):
You can put this solution on YOUR website!
D=RT
40=(R+5)T OR T=40/(R+5)
20=(R-5)T OR T=20/(R-5)
BECAUSE THE TIMES ARE THE SAME SET THESE TWO EQUATION EQUAL.
40/(R+5)=20/(R-5)
CROSS MULTIPLY
40(R-5)=20(R+5)
40R-200=20R+100
40R-20R=100+200
20R=300
R=300/20
R=15 MPH FOR THE SPEED OF THE BOAT IN STILL WATER.
PROOF:
40/(15+5)=20/(15-5)
40/20=20/10
2=2