NEW HERE? LEARN MORE
closeclose
Last visit was: 01 May 2024, 17:52 It is currently 01 May 2024, 17:52
Decision Tracker
My Rewards
New posts
New comers' posts

Close
GMAT Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Go to My Error Log Learn more
Close
Request Expert Reply
Confirm Cancel

A 100 meter wide river can be crossed straightly, in 4 minut

SORT BY:
Date
Kudos
Tags:
Find Similar Topics 
Show Tags
Hide Tags
User avatar
S
Mugdho
Manager
Manager
Joined: 27 Feb 2019
Posts: 95
Own Kudos [?]: 134 [14]
Given Kudos: 495
Send PM
A 100 meter wide river can be crossed straightly, in 4 minut [#permalink] New post   26 Dec 2021, 22:04
14
Bookmarks
00:00
A
B
C
D
E

Difficulty:

95% (hard)

Question Stats:

9% (02:01) correct 91% (01:33) wrong based on 69 sessions
Hide Show timer Statistics
A 100 meter wide river can be crossed straightly, in 4 minutes when there is no stream and 5 minutes when there is stream. What is the speed of the stream ?

a) 10 meter/minute
b) 15 meter/minute
c) 20 meter/minute
d) 5 meter/minute
e) None

Posted from my mobile device
ShowHide Answer
Official Answer
B
User avatar
L
KarishmaB
Tutor
Joined: 16 Oct 2010
Posts: 14847
Own Kudos [?]: 65029 [4]
Given Kudos: 429
Location: Pune, India
Send PM
A 100 meter wide river can be crossed straightly, in 4 minut [#permalink] New post   26 Dec 2021, 22:31
2
Kudos
2
Bookmarks
Expert Reply
Mugdho wrote:
A 100 meter wide river can be crossed straightly, in 4 minutes when there is no stream and 5 minutes when there is stream. What is the speed of the stream ?

a) 10 meter/minute
b) 15 meter/minute
c) 20 meter/minute
d) 5 meter/minute
e) None

Posted from my mobile device


This is what the question means - if you cross the 100 m side river directly in still water, you would take 4 mins.
So your speed (without any external influence) = 100/4 = 25 meter/min

But the stream sways you away every moment as you swim and hence makes you cover extra distance and makes you travel diagonally (as shown in the diagram)
So distance covered in 5 mins = 25*5 = 125 meters

Attachment:
Screenshot 2021-12-27 at 10.57.27.png
Screenshot 2021-12-27 at 10.57.27.png [ 19.37 KiB | Viewed 2525 times ]


This is a right triangle of sides ratio 3-4-5 so the third side is 75 meters (distance between point of 'still water' and point of 'with stream').

Hence the stream moved 75 meters in 5 mins. Imagine one water molecule that stuck to you when you entered the river and then acted upon you till you reached the other end. That molecule travelled 75 metres in 5 mins.

Speed of the stream = 75/5 = 15 meters/min

Answer (B)
User avatar
S
Mugdho
Manager
Manager
Joined: 27 Feb 2019
Posts: 95
Own Kudos [?]: 134 [0]
Given Kudos: 495
Send PM
Re: A 100 meter wide river can be crossed straightly, in 4 minut [#permalink] New post   27 Dec 2021, 06:02
KarishmaB wrote:
Mugdho wrote:
A 100 meter wide river can be crossed straightly, in 4 minutes when there is no stream and 5 minutes when there is stream. What is the speed of the stream ?

a) 10 meter/minute
b) 15 meter/minute
c) 20 meter/minute
d) 5 meter/minute
e) None

Posted from my mobile device


This is what the question means - if you cross the 100 m side river directly in still water, you would take 4 mins.
So your speed (without any external influence) = 100/4 = 25 meter/min

But the stream sways you away every moment as you swim and hence makes you cover extra distance and makes you travel diagonally (as shown in the diagram)
So distance covered in 5 mins = 25*5 = 125 meters

Attachment:
Screenshot 2021-12-27 at 10.57.27.png


This is a right triangle of sides ratio 3-4-5 so the third side is 75 meters (distance between point of 'still water' and point of 'with stream').

Hence the stream moved 75 meters in 5 mins. Imagine one water molecule that stuck to you when you entered the river and then acted upon you till you reached the other end. That molecule travelled 75 metres in 5 mins.

Speed of the stream = 75/5 = 15 meters/min

Answer (B)



One doubt in the solution: if someone swims straightly then he will go 125 meter in 5 minutes in the still water.

In the second case, when he is going with his own speed + stream speed, he is still going 125 meter in 5 minutes.

How is this possible ?
avatar
B
ankurgarg2410
Intern
Intern
Joined: 19 Dec 2021
Posts: 7
Own Kudos [?]: 18 [0]
Given Kudos: 0
GRE 1: Q170 V152
Send PM
A 100 meter wide river can be crossed straightly, in 4 minut [#permalink] New post   Updated on: 27 Dec 2021, 09:34
Mugdho wrote:
KarishmaB wrote:
Mugdho wrote:
A 100 meter wide river can be crossed straightly, in 4 minutes when there is no stream and 5 minutes when there is stream. What is the speed of the stream ?

a) 10 meter/minute
b) 15 meter/minute
c) 20 meter/minute
d) 5 meter/minute
e) None

Posted from my mobile device


This is what the question means - if you cross the 100 m side river directly in still water, you would take 4 mins.
So your speed (without any external influence) = 100/4 = 25 meter/min

But the stream sways you away every moment as you swim and hence makes you cover extra distance and makes you travel diagonally (as shown in the diagram)
So distance covered in 5 mins = 25*5 = 125 meters

Attachment:
Screenshot 2021-12-27 at 10.57.27.png


This is a right triangle of sides ratio 3-4-5 so the third side is 75 meters (distance between point of 'still water' and point of 'with stream').

Hence the stream moved 75 meters in 5 mins. Imagine one water molecule that stuck to you when you entered the river and then acted upon you till you reached the other end. That molecule travelled 75 metres in 5 mins.

Speed of the stream = 75/5 = 15 meters/min

Answer (B)



One doubt in the solution: if someone swims straightly then he will go 125 meter in 5 minutes in the still water.

In the second case, when he is going with his own speed + stream speed, he is still going 125 meter in 5 minutes.

How is this possible ?


The speed of the stream will have no effect on the speed of the one crossing the stream, as it is acting in the perpendicular direction. The only effect stream has is in increasing the distance one has to cover to be able to cross.

Edit: I stand corrected. Thanks, Regor60!
Even if it seems that the distance is increasing, the swimmer's efforts are directed in the x axis. All the efforts in the y axis comes from the stream, which means from the swimmer's perspective, distance and speed remain same as they were in the case of lack of stream.

Originally posted by ankurgarg2410 on 27 Dec 2021, 08:17.
Last edited by ankurgarg2410 on 27 Dec 2021, 09:34, edited 1 time in total.
User avatar
G
Regor60
Senior Manager
Senior Manager
Joined: 21 Nov 2021
Posts: 437
Own Kudos [?]: 213 [1]
Given Kudos: 345
Send PM
A 100 meter wide river can be crossed straightly, in 4 minut [#permalink] New post   27 Dec 2021, 09:21
1
Kudos
Assuming the man is swimming perpendicular to the current, the only reason it would take him longer to cross the river is if he CHOSE to swim slower.

So the man isn't swimming perpendicular to the current. He's swimming 25 m/s against the current at an angle that allows him to go straight across at 20 m/s, a component of his speed offsetting the current.

This is a simple right triangle with 25 m/s as hypotenuse and 20 m/s as horizontal leg.

Speed of current is then (25^2-20^2)^1/2 = 15 m/s.



Posted from my mobile device
GMAT Club Bot
gmatclubot
A 100 meter wide river can be crossed straightly, in 4 minut [#permalink]
27 Dec 2021, 09:21
Moderators:
Bunuel
Math Expert
92987 posts
yashikaaggarwal
Senior Moderator - Masters Forum
3137 posts

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne
Main navigation
GMAT Resources
Partners
MBA Resources
www.gmac.com|www.mba.com | | GMAT Club Rules | Terms and Conditions