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Question 515481: BOATING TRIP: Cesar starts out on a boating trip at 8 am. Cesar's boat can go 20 miles per hour in still water. How far downsream can Cesar go if the current is 5 miles per hour and he wishes to go down and back in 4 hours?
Answer by oberobic(2304)   (Show Source):
You can put this solution on YOUR website! d = r*t is the basic distance equation.
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r = 20 mph in still water
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c = 5 mph is the current in the river
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going upstream, the speed = r-c = 20-5 = 15
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going downstream, the speed = r+c = 25
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d = d each way (that is, he returns to the same point he started).
The roundtrip = 2d.
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d/r = t
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he wants to make the whole trip in 4 hr
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d/15 + d/25 = 4 hr
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(25d + 15d) / (15*25) = 4
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cross multiply
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40d = 4*15*25
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d = 4*15*25/40
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d = 75/2 = 37 1/2 miles
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Always check your answer.
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In this case, what can we check?
Time.
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37.5 miles/ 15 mph = 2.5 hr going upstream (against the current)
37.5 miles/ 25 mph = 1.5 hr going downstream (with the current)
2.5+1.5= 4 hr
Correct.
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Done.
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