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A boat makes a 120-mile trip downstream in 3 hours but makes the return trip in 4 hours.. . What is the rate of the current?. A) 3 mph. B) 4 mph. C) 5 mph.

User MillsJROSS
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7.3k points

2 Answers

1 vote
Option c is correct answer because
Let c = rate of current
Let b = rate of boat in still water
Then b+c = rate downstream
b-c = rate upstream
So 3(b+c) = 120 = b+c = 40
And 4(b-c) = 120 = b-c = 30
So 2b = 70 ... b = 35 mph speed in still water
So c = 5 mph = current speed
User Brandizzi
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8.9k points
5 votes
Let the still water boat speed be b, and let the rate of the current be c.
3(b + c) = 120 ...........(1)
4(b - c) = 120 ............(2)
Multiply (1) by 4 and multiply (2) by 3, to get:
12b + 12c = 480 .......(3)
12b - 12c = 360 ........(4)
Add equations (3) and (4):
24b = 840
b = 35 mph
Plugging the value of b into equation (3) and solving for c, gives the value of 5 for c.
Therefore answer choice C) 5 mph is correct.
User Fieldtensor
by
8.2k points

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