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# The current in a river is 4 mph. A boat can travel 20 mph in still wat

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The current in a river is 4 mph. A boat can travel 20 mph in still wat [#permalink]

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24 Jan 2016, 05:38
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The current in a river is 4 mph. A boat can travel 20 mph in still water. How far up the river can the boat travel if the round trip is to take 10 hours?

A. 69 miles
B. 88 miles
C. 96 miles
D. 100 miles
E. 112 miles
[Reveal] Spoiler: OA

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Re: The current in a river is 4 mph. A boat can travel 20 mph in still wat [#permalink]

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24 Jan 2016, 06:48
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Upstream Speed = 20-4=16 mph
Downstream Speed = 20+4 = 24 mph

D/16 + D/24 = 10 hours
Solving for D we get D=96

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Re: The current in a river is 4 mph. A boat can travel 20 mph in still wat [#permalink]

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28 Jan 2016, 22:55
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Expert's post
Hi All,

This question is a multi-step rate question, so you'll need to use the Distance Formula, but you can actually go about solving it in a couple of different ways.

From the prompt, we know that the rate "with" the current is 24 miles/hour and the rate "against" the current is 16 miles/hour. Since 24/16 = 1.5, there's an interesting way to convert this information into a 'unit' that we can use to our advantage...

IF...we traveled upstream for 24 miles, then downstream for 24 miles, we would spend....

"with" the current
D = (R)(T)
24 = (24 mph)(T)
1 hour = T

"against" the current
D = (R)(T)
24 = (16 mph)(T)
1.5 hours = T

TOTAL Time = 1 + 1.5 = 2.5 hours to travel 24 miles in each direction. This data is useful, since the question asks how far the boat can travel "with" the current if the TOTAL trip takes 10 hours.

10 hours/2.5 hours = 4 "blocks" of 24 miles

(4)(24) = 96 miles in each direction.

[Reveal] Spoiler:
C

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Re: The current in a river is 4 mph. A boat can travel 20 mph in still wat [#permalink]

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24 Jun 2017, 14:39
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Another approach is using the term 'round trip' and equating distances

$$R1*T1 = R2*T2$$
Given $$T1 + T2 = 10$$
Speed upstream is 16 (deduct opposing current)
Speed downstream is 24 (add current)

$$24*T1=16(10-T1)$$
$$T1=4$$

Hence $$D1 = R1*T1 = 24*4 = 96$$
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Re: The current in a river is 4 mph. A boat can travel 20 mph in still wat [#permalink]

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24 Jun 2017, 18:02
Bunuel wrote:
The current in a river is 4 mph. A boat can travel 20 mph in still water. How far up the river can the boat travel if the round trip is to take 10 hours?

A. 69 miles
B. 88 miles
C. 96 miles
D. 100 miles
E. 112 miles

let t=upstream hours
24 mph*(10-t) hours=16t miles
t=6 hours
6 hours*16 mph=96 upstream miles
C

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Re: The current in a river is 4 mph. A boat can travel 20 mph in still wat [#permalink]

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24 Jun 2017, 18:50
Let D be the distance upstream.
D/20-4 + D/20+4 = 10
D(1/16 + 1/24) = 10
D(3+2/48) = 10
D/48 = 2
D = 96.
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Re: The current in a river is 4 mph. A boat can travel 20 mph in still wat [#permalink]

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05 Dec 2017, 08:28
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Expert's post
Top Contributor
Bunuel wrote:
The current in a river is 4 mph. A boat can travel 20 mph in still water. How far up the river can the boat travel if the round trip is to take 10 hours?

A. 69 miles
B. 88 miles
C. 96 miles
D. 100 miles
E. 112 miles

Boat's speed UPSTREAM = 20mph - 4mph = 16mph
Boat's speed DOWNSTREAM = 20mph + 4mph = 24mph

(travel time UPSTREAM) + (travel time DOWNSTREAM) = 10 hours

time = distance/speed
We know the speeds, but not the distance each way
So, let d = distance EACH EACH

Our word equation becomes....
d/16 + d/24 = 10
Eliminate the fractions by multiplying each side by 28 (the least common multiple of 16 and 24)
When we do this, we get: 3d + 2d = 480
Simplify: 5d = 480
Solve: d = 96

So, the distance EACH WAY is 96 miles

Cheers,
Brent
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Re: The current in a river is 4 mph. A boat can travel 20 mph in still wat [#permalink]

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22 Dec 2017, 08:31
GMATPrepNow wrote:
Bunuel wrote:
The current in a river is 4 mph. A boat can travel 20 mph in still water. How far up the river can the boat travel if the round trip is to take 10 hours?

A. 69 miles
B. 88 miles
C. 96 miles
D. 100 miles
E. 112 miles

Boat's speed UPSTREAM = 20mph - 4mph = 16mph
Boat's speed DOWNSTREAM = 20mph + 4mph = 24mph

(travel time UPSTREAM) + (travel time DOWNSTREAM) = 10 hours

time = distance/speed
We know the speeds, but not the distance each way
So, let d = distance EACH EACH

Our word equation becomes....
d/16 + d/24 = 10
Eliminate the fractions by multiplying each side by 28 (the least common multiple of 16 and 24)
When we do this, we get: 3d + 2d = 480
Simplify: 5d = 480
Solve: d = 96

So, the distance EACH WAY is 96 miles

Cheers,
Brent

I really like your algebraic approach "word equation".
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Re: The current in a river is 4 mph. A boat can travel 20 mph in still wat   [#permalink] 22 Dec 2017, 08:31
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