Aptitude - Boats and Streams - Discussion

5. 

In one hour, a boat goes 11 km/hr along the stream and 5 km/hr against the stream. The speed of the boat in still water (in km/hr) is:

[A]. 3 km/hr
[B]. 5 km/hr
[C]. 8 km/hr
[D]. 9 km/hr

Answer: Option C

Explanation:

Speed in still water = 1 (11 + 5) kmph = 8 kmph.
2

Video Explanation: https://youtu.be/KQX_mA3tcVA


Ashok said: (Jul 19, 2011)  
From where do this 1/2 came from?

Vivek Patidar said: (Sep 5, 2011)  
Because in the condition of still position of water the speed one side and opposite side of water will be equal thats why we will take average of both speed.

Manjit Singh said: (Sep 15, 2011)  
Here speed along the stream and against the stream is given. Then how can we assume water to be steady (both the ways). Then why this 1/2 has come?

Debapriya Dandapat said: (Sep 17, 2011)  
Upstream relative speed is u + v=11km/hr

Downstream speed is u-v=5

Where u= speed of boat in still water and v is speed of stream

So adding two equations u+v + u-v =11+5
2u=16

So, u=8.

Ajit Jha(Iit Kharagpur) said: (Dec 30, 2011)  
This Q is wrong buddy...
Here in stream dir. it takes 1 hour and again in downstream it takes 1 hour.
Then..
x+y=11/1=11------(1)
x-y=5/1=5----(2)
fm eq. 1 & 2.. we get..
x=8 km/h

Saloni Nautiyal said: (Feb 25, 2012)  
Formula for speed of boat in still water=[(X+Y)+(X-Y)]/2
where X=SPEED OF BOAT IN STILL WATER
Y=SPEED OF STREAM/CURRENT
THEREFORE, [(11+5)+(11-5)]/2=11
ANSWER=11
ANSWER IS NOT IN THE OPTIONS

Kottitheking said: (Sep 9, 2012)  
Upstream relative speed is u + v=11km/hr

Downstream speed is u-v = 5

Where u = speed of boat in still water and v is speed of stream

Then adding two equations u+v + u-v =11+5
2u=16

Finally, u=8.

Brinda Iyer said: (Mar 10, 2013)  
But if in the same question distance traveled is asked, then what is the formula?

Ritha Epsiba said: (Mar 13, 2013)  
If the speed of a boat in still water is u km/hr and the speed of the stream is v km/hr, then:

Speed downstream = (u + v) km/hr.

Speed upstream = (u - v) km/hr.

Arun.D said: (Jun 3, 2013)  
Downstream=11km/hr.
Upstream=5km/hr.
Speed in still water = 1/2(a + b) km/hr.

a = upstream.

b = downstream.

Speed = 1/2(11+5)km/hr.
= 1/2(16)km/hr.
= 8km/hr.

Mina said: (Jan 7, 2015)  
What is u and v?

Subaram said: (Jan 8, 2015)  
Here direction along the stream called downstream and against the stream called upstream. In this question they gave along the stream 11 kmhr against the stream 5 kmhr. So speed still in water = 1/2 (a+b) , here a downstream be up stream as for formula.

Naresh said: (Apr 28, 2015)  
Given that,

Speed in the downstream (d) = 11 km/hr.

Speed in the upstream (u) = 5 km/hr.

Speed of the Boat in still water = d + u/2.

=> 11 + 5/2 = 16/2.

=> 8 km/hr.

Ramachandran said: (May 28, 2015)  
Hello guys how would you find:

11 km/hr is Downstream.
5 km/hr is Upstream.

Suchita said: (Aug 9, 2015)  
Because in question they have mentioned speed of boat along the stream (i.e downstream) is 11km/hr and against the stream (i.e upstream) is 5km/hr.

Nikul Rana said: (Nov 1, 2015)  
Speed downstream = X+Y = 11 kmph;

Speed upstream = X-Y = 5 kmph;

Speed of the still water = (X+Y)+(X-Y)*1/2 = 8 kmph.

Priya said: (Dec 10, 2015)  
What is this? Am not clear, I am late pick up person, say clearly guys.

Nija said: (Jan 18, 2016)  
In this question, it is not mentioned whether 1 hour each taken by downstream and upstream. So how can we solve this questions ? Please am confused.

Ayan said: (Apr 13, 2016)  
The question is not clear. It says in one hour both travel upstream as well as downstream. How is it explain me?

Shruti said: (Jun 22, 2016)  
How come the speed becomes 11 kmph and 5 kmph when only distance is given, and boat travels both the distances in total 1 hour?

I think either the question is wrong or 8 is not the answer.

Shudhu said: (Jul 31, 2016)  
Downstream so, x + y = 11.
Upstream so, x - y = 5.

Solve equation 1 and 2.

You get x =7.5.

So, nearest answer is 8.

Sara said: (Jan 2, 2017)  
x = speed of stream still in water.
y = speed of stream.

downstream = Distance/ X+Y = Time,
= 11/ X+Y = 1hour.

Upstream= Distance/ X-Y = Time.
= 5/X-Y = 1hour.

Cross multiply

11 = X + Y -----(1)eq
5 = X - Y ------(2)eq

We need x, so cancel y.
2X = 16,
X = 8kmph.

Aman Kumar said: (Jan 27, 2017)  
B = Speed of boat.
w = Speed of water.

NOW
B + W=11 (with the stream)
B - W=5 (opp to stream)
----------------
2B =16,
B =16/2,
= 8 KMPH.

Suresh said: (Feb 26, 2017)  
A boats cover 1 km in 10 minute with stream and 1 km in 20 minute against stream now speed of boat in still water will be?

Please solve it.

Audry said: (Mar 15, 2017)  
Let U= speed of boat in still water and v=speed of stream then;

Upstream speed, U - V = 5km/hr,
Downstream speed, U + V = 11km/hr,

Eqt,
U + V + U - V = 11 + 5
2U = 16
U = 16/2 = 8km/hr.

Satish said: (Jun 17, 2017)  
D=11 U=5.
(D+U)/2=x.
11+5/2=X.
16/2=X.
8=X (stream speed).

Bhavesh said: (Aug 12, 2017)  
What is this same Q is like in 3 hours a boat goes 11 km along the stream and 5 km against the stream the speed of the boat in still water is?

Kamala Kant said: (Sep 11, 2017)  
Do you have the concept of eqillibrium? then say
if , (11/x +y) + (5/x-y) =3,
x=?

Yamini said: (Oct 26, 2017)  
If the speed downstream is a km/hr and speed upstream is b km/hr then,
Speed in still water=1/2(a+b)km/hr.

Rz Reddy said: (Nov 4, 2017)  
Downstream(D).
Upstream so(U).
D+U = 11.
D-U = 5.
2D = 16.
D = 16/2.
D = 8km/hr.

Nurun Nobi Khokon said: (May 23, 2018)  
Let, downstream (with stream) speed= x.
upstream (against) speed is= y.
Now,
x = speed of the boat+speed of the water.
y = speed of the boat-speed of water.
-----------------------------------------------------------
x+y = 2*speed of the boat.

So, speed of the boat = (x+y)/2 [downstream speed+upstream speed].

Hari said: (Jun 11, 2018)  
For finding the speed of the boat in still water = (upstream-downstream/2).
For finding water speed = (upstream+downstream/2).

Shandeepsamy said: (Jul 2, 2018)  
Where this 1/2 came from? Please explain.

Darkele said: (Jul 4, 2018)  
Note, there is no use of the 1 hour given because the upstream and downstream is sufficient to get the speed of the boat in still water.

Vinau said: (Sep 5, 2018)  
X+Y=11,
X-Y=5,
X+Y+X-Y=11 + 5,
2X=16.

Then, X=8.

A.K.Balakrishnan said: (Sep 13, 2019)  
@All.

Follow this method for all boat question;

Downstream means boat towards the down in this situation we have to find the speed of boat and speed of stream.

Let x be boat speed.
Let y be stream speed.

1) Down stream speed = D/T(downstream time) = x+y

note: distance will be the same as both the streams.

2) Upper stream speed = D/T (upper stream time) = x-y.

Thank you.

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