Aptitude - Boats and Streams - Discussion

13. 

Speed of a boat in standing water is 9 kmph and the speed of the stream is 1.5 kmph. A man rows to a place at a distance of 105 km and comes back to the starting point. The total time taken by him is:

[A]. 16 hours
[B]. 18 hours
[C]. 20 hours
[D]. 24 hours

Answer: Option D

Explanation:

Speed upstream = 7.5 kmph.

Speed downstream = 10.5 kmph.

Total time taken = 105 + 105 hours = 24 hours.
7.5 10.5


Ramya said: (May 2, 2011)  
How do you arrive at upstream and down stream speed.

Bhavana said: (Jul 2, 2011)  
How do you calculate upstream and downstream speed?

Bhavana said: (Jul 2, 2011)  
Speed upstream is 9+1.5 and downstream is 9-1.5.

I think generally in stream sums we should calculate first the upstream and the downstream values.

Kau said: (Sep 18, 2011)  
You are wrong Bhavana, you have written opposite.

Raju said: (Nov 5, 2011)  
Time taken for up stream = 105/(9-1.5)=14h
Time taken for down stream =
105/(9+1.5)=10h Total time taken= 14+10=24h

Rakesh Kiit said: (Sep 3, 2012)  
Average speed During the Journey = (2*(9+1.5)*(9-1.5))/(9+1.5)+
(9-1.5)

so,Average speed=8.75 kmph

so,Total time= total distance/average speed
=(105*2)/8.75
= 24 hours.

Abhi Poojary said: (Aug 18, 2013)  
Just calculate downstream. 10 hours. Upstream has to be more than 10. So only 24 possible.

Gego Georoge said: (Mar 4, 2014)  
I have a doubt, 9 km in question is speed of Boat, and a man rows 105 km but his speed is not given, and we have to find time taken by man.

Sidheswar Sahoo said: (Apr 3, 2014)  
9 km/hr is the speed of boat in standing water.

Boat has to move twice the distance of 105km to reach at same point.

Then (105/9)+(105/9) = 23.33333333333333333.

Then It should be 24hr.

Saran said: (May 8, 2014)  
Speed of boat in still water is 9km how could you consider it as upstream speed similarly to downstream also they couldn't mention anything like that. Please explain.

Azar said: (Sep 23, 2014)  
On what basis Upstream and down stream speed is calculated?

Chandini said: (Sep 30, 2014)  
Speed of boat in still water = 9 kmph.
Speed of stream = 1.5 kmph.

Given distance from boat to a place is 105 km.

Consider a man want to go to that place, means he want go in opposite direction of boat( away from boat).

So we take upstream = sp(boat)-sp(stream) = 9-1.5 => 7.5kmph.
Time taken to travel in upstream is 105/7.5 = 14 hours.

By returning he travel towards boat sp(boat)+sp(stream) = 9+1.5 =>10.5kmph.
Time taken to travel in downstream is 105/10.5 = 10 hours

Total time taken to travel == 14+10 = 24 hours.

Kalaivani said: (Sep 15, 2016)  
How to know downstream and upstream?

Vinod said: (Feb 9, 2017)  
How? explain this.

Atul Kumar Singh said: (Feb 20, 2017)  
Guys, it's very simple you don't need to find upstream or downstream speed you can the total time like this.

Let T be total time and t1=return time and t2=going time and D = be the distance(105) x=still water speed of man and y=stream speed so,

T = t1+t2
T = (D/x+y)+(D/x-y) {definitely one direction will be upstream n oppsite dir will b downstream}
T = D{(2x)/(x^2 - y^2).

Solution.
T = (105 *2*9)/(9^2-1.5^2).
T = (1890)/(78.75),
T = 24 Hours.

Divyanjali said: (Apr 9, 2017)  
The upstream and downstream speeds are calculated on relative motion.

If two bodies are moving in the same direction their speed is substracted and if they're moving in an opposite direction they are added.

Priti Kumari said: (Mar 11, 2018)  
How did we get that once it was upstream and next time it was downstream?

Dhiraj Sinha said: (Mar 14, 2018)  
Guys look current or speed of boat mean v and in still water or stationary means u.

So for down u+v and for upstream u-v. So calculate the time by t=d/s and add both the time.

Rajesh said: (Jun 1, 2018)  
As we know speed of the boat is U and the speed of the stream is V,

For Upstream speed=U-V (9-1.5).
For downstream speed = U+V (9+1.5).
what he mentioned is he rows against the stream with a distance of 105km.
in order to find the total time took i.e., T=D/S.
so, T=105/10.5 + 105/7.5 (downstream) = 24hrs.

Aman said: (Aug 16, 2018)  
U.S = 1/2(P+Q).
D.S = 1/2(P-Q).

Vijay . C said: (Nov 7, 2019)  
Upstream speed = 9 - 1.5 = 7.5kmph.

Downstream speed = 9+1.5 = 10.5kmph.

D = S * T
D = 105kmph, S in upstream = 7.5kmph.

T has taken in upstream = 105/7.5.

Similarly,

T has taken in downstream = 105/10.5.

Total time taken = ( 105/7.5.)+ (105/10.5).

Multiply by 10,

T = (105*10/75) + (105*10/105).

T = 14 + 10.

T = 24hrs.

Ranvijay said: (Oct 23, 2020)  
How it is taken 7.5 for upstream? please explain.

Rajat Kumar Kara said: (May 16, 2021)  
Guys, It's given the Speed of a boat in standing water and asking for the time taken by man to travel upstream and downstream. How you guys calculate upstream and downstream for the man?

Can't get it, Please explain.

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