Aptitude - Boats and Streams - Discussion

2. 

A man's speed with the current is 15 km/hr and the speed of the current is 2.5 km/hr. The man's speed against the current is:

[A]. 8.5 km/hr
[B]. 9 km/hr
[C]. 10 km/hr
[D]. 12.5 km/hr

Answer: Option C

Explanation:

Man's rate in still water = (15 - 2.5) km/hr = 12.5 km/hr.

Man's rate against the current = (12.5 - 2.5) km/hr = 10 km/hr.


Samarth said: (Nov 13, 2010)  
As man is not travelling against anything, hence why is rate of current subtracted from 15 in first step? I think the first step should be final step.

Tushar said: (Dec 5, 2010)  
No, Man is travelling with water when it is 15 km, so if we subtract 2.5, it gives us man's own speed and then again subtracting gives us upstream speed

Pinky said: (Feb 24, 2011)  
Please explain me, why man's rate subtract form the current,

I'm not understanding.

Ashwini said: (Feb 27, 2011)  
I'm not understanding

Balaji said: (Mar 18, 2011)  
Let me explain:

First of all the problem has the data,
speed of man is with current i.e., 15km/hr.
here we can compare it with down stream i.e., speed of boat with the stream.
if we want to take man's speed only we got to substract the speed of current i.e., 2.5 km/hr.
therefore, 15-2.5 = 12.5 km/hr, which is actually man's speed.

In the question they asked us to determine the speed of the man against the current. Here we will compare it with up stream.
i.e. speed of boat against the stream.
there fore, speed of man - speed of current
=12.5-2.5=10km/hr

Khaleel said: (Mar 29, 2011)  
You are right samarth.

Geetha said: (Jun 16, 2011)  
Thank you. Now I understand clearly.

Sravya said: (Sep 3, 2011)  
Let speed of man be 'a'
speed of current be 'b'
given man's speed with current (a+b)=15 km\hr nd speed of current is 2.5
substitute in abv eqn (a+2.5)=15
a=15-2.5
a=12.5
but problem is abt finding man's speed against current i,e
(a-b)=12.5-2.5
=10

Anup Gupta said: (Sep 23, 2011)  
No upstream speed can be any one less than 12.5. I. E. Against the stream. Question is not that what will be the speed if he will move against the stream at 2.5.

Hitesh Bulani said: (Dec 7, 2011)  
Thanku balaji.

Mani said: (Jan 5, 2012)  
Here they did not mention that is/clearly not questioned that this is a boad and stream problem.

Suraj said: (Feb 19, 2012)  
Thanks balaji.

Prasad said: (May 11, 2012)  
How do we know the man is traveling in boat with out mention in the question?

Raj said: (Jun 2, 2012)  
The speed of man is 15 including the speed of current.
Actual speed of man is 12. 5,
And now against the current 12. 5-2. 5 (speed of current) =10.

Sapna said: (Sep 10, 2012)  
Solve acc to formula,
We have

Current=1/2(a-b).

So,

2.5=1/2(15-b).

2.5*2=15-b.

b=10km/hr.

Karthik said: (Nov 24, 2012)  
The actual speed is 12.5. Due to downstream it becomes 15 (adding 2.5) and due to upstream it becomes 10 (subtracting 2.5). Simple no confusions.

Atuljain said: (Jun 1, 2013)  
Downstream = speed of the man + speed of the stream.
15 = x+2.5.
x = 12.5.

Upstream = speed of the man - speed of the stream.
x = 12.5-2.5.
x = 10.

Arun.D said: (Jun 3, 2013)  
Man's speed with the current = 15 km/hr.
Speed of the current = 2.5 km/hr.
Man's speed against the current=?

Downstream = (u+v)km/hr.

u+v = 15.
v = 2.5.

u+2.5 = 15.

u = 15-2.5.

u = 12.5km/hr.

Upstream = (u-v)km/hr.
= (12.5-2.5)km/hr.
= 10km/hr.

Ramakrishna said: (Sep 26, 2013)  
Given that mans speed with current means mans speed+current = 15, So here current is given from which we get actual speed of man is 12.5km/hr, Then he asked against the current so the answer will be 12.5 - 2.5=10km/hr.

Vijaya said: (May 1, 2014)  
Hi. Guyz. I have one doubt. ? how we know. The given question is upstream or downstream. I can't understand that. If you don't mind can you please tell me that.

Kush said: (Aug 4, 2014)  
hi ! @Vijaya.

In question, it will be already provided the keyword like upstream or downstream and With stream or Against Stream.

Bhavani said: (Sep 23, 2014)  
With stream = a+b(speed of d object in still water +speed of d stream) => Down stream.

Against stream = a-b(speed of d object in d still water - speed of d stream) => up stream.

Shweta Singh said: (Jan 5, 2015)  
Given:

Speed downstream, a = 15 km/h.
Rate of current, v = 2.5 km/h.

We need to find the value of b, i.e. speed upstream (Speed against the current).

As we know that, speed downstream, a = u+v.
Substituting the value of a and v;

15 = u+2.5.
u = 12.5 km/h.

Now as u = 12.5 km/h and v = 2.5 km/h.
So, speed upstream, b = u-v.

b = 12.5-2.5.
b = 10 km/h answer.

So the right answer is 10 km/h.

Naresh said: (Apr 28, 2015)  
A man speed with the current means A man moving along with water, means it is downstream(d) speed = 15 km/hrs.

Rate of current or stream (s) = 2.5 km/hrs.

So,

From the downstream formula (d) = b + s.

We know the d and s values, so substitute those values in the above formula and obtain the b value i.e.

b = 15-2.5 ==> 12.5 kh/hrs.

So we need to calculate the man's speed against the water it will happen in upper stream (u) = b - s.

=> 12.5-2.5 ==> 10 km/hrs.

Sahasra said: (May 15, 2015)  
Let me clear first. What is meant by "Current"? Please any 1 explain it?

Akki said: (Nov 22, 2015)  
This is for @Vijaya.

Suppose there is a river which is flowing from left to right. And if you are swimming left to right. Your speed + River's speed i.e. downstream (with stream).

And if you are swimming from right to left. Your speed - Speed of river upstream (against the river i.e since you are swimming against the upstream speed.

Devi said: (Feb 8, 2016)  
Hai I'm not understanding this type of boats and streams problems I don't know what is meant by current speed like that please any one can explain me?

Vishal said: (Feb 17, 2016)  
Why was the speed subtracted. I think it should be added.

For downstream. If we consider velocity of man and velocity of water as vector quantities.

Then velocity of man we are at the stream i.e Stream = Velocity of man we are at ground - Velocity of stream we are at ground.

15 I = x - 2.5 I.

x = 17.5 I.

Can anyone explain my doubt?

Rucha said: (Mar 6, 2016)  
Can anybody explain me the meaning of question clearly. It's to confusing for me.

Mani said: (Sep 6, 2016)  
Man's speed with the current means downstream speed i.e. 15 km/ph.

The speed of current means speed of stream i.e 2.5 km/ph.

Man's speed against the current means upstream.

Now we have to find upstream speed.

Speed in current = 1/2 (downstream speed - upstream speed).

2.5 = 1/2 (15 - x).

5 = 15 - x.

-10 = -x.

x = 10 km/ph.

Ravi said: (Sep 14, 2016)  
Well said @Mani.

Roshni Yadav said: (Oct 7, 2016)  
Thank you @Balaji.

Arjun said: (Dec 10, 2016)  
Well explain by Mani and Balaji.

Ramu said: (Dec 18, 2016)  
Thank you @Balaji.

Shahid said: (Mar 24, 2017)  
Very good explanation, thank you all.

Arun said: (Mar 31, 2017)  
Stream direction is same while man's direction is opposite in the case of upstream.

Then why we subtract stream instead of the man's speed?

Swathi said: (Jun 6, 2017)  
Why can't we use the formula 1/2 (a-b) to get the upstream value directly?

That time the answer would be 20 km/hr. Please tell me.

Shyam said: (Jun 7, 2017)  
@Vishal.

15 is the speed of the man "with" speed of current i.e., 15 is the speed of man in downstream.
That means M + S = 15; S = 2.5.
Finally, man's speed =15-2.5 = 12.5.

Sibani said: (Aug 11, 2017)  
Is both the terms rate of stream and speed of the current are same.
If both are same I am getting 12.5 as an answer for this question, can anyone clarify it?

Mahima said: (Feb 10, 2018)  
Thank you @Balaji. You explained it very clearly.

Deepika said: (Feb 28, 2018)  
Sir, I have one doubt.

Given downstream speed (i.e speed of boat with stream)is 15 km/hr .asked ..upstream speed is( boat speed(u)- stream speed(v)).

Here to find the boat speed we use boat speed= downstream speed - stream speed.what should we do if upstream speed( speed of boat against stream) is given in the question.

Should we do like this, boat speed=upstream speed+ stream speed.

Akash R said: (Mar 7, 2018)  
Thank you @Balaji.

Peauts said: (Apr 8, 2018)  
You are right @Deepika

a=u+v(speed of downstream/speed of man with the stream(i.e)he is going along with the stream)
u=a-v (speed of the man alone)=12.5
b=u-v
if b is given:
put b=u-v
So, u=b+v.

Abc said: (Apr 9, 2018)  
Not getting. Please, someone, help me to get it.

Hari said: (Jun 11, 2018)  
Speed in current =15,
Current speed =2.5,
Upstream =15-2.5=12.5,
Downstream =15+2.5=17.5.

Shripad said: (Jun 28, 2018)  
Thanks @Atuljain.

Sijohn Joy said: (Aug 22, 2018)  
d=15 y=2.5.
u=?.

15-u/2=2.5,
15-u=5,
u=15-5=10.

Paddu said: (Sep 4, 2018)  
Thank you @Mani.

Yogi said: (Sep 30, 2018)  
I am not understanding this. Please explain me.

Ankita Chandra said: (Feb 12, 2019)  
Man's speed + speed of current = 15,
Man's speed + 2.5 = 15,
Man's speed = 15 - 2.5 = 12.5,
Man's rate against the current i.e. difference between their speed ( u-v).
Man's speed - speed of current.
= 12.5 - 2.5,
= 10.

Ganga said: (May 21, 2019)  
Man's speed with current=15.

i.e., man's speed + current speed =15 km,
Now we want man's speed alone, so;

Man's speed = 15-currrent speed.
i.e.,man's speed = 15-2.5 = 12.5.

Now calculate Man's speed against the current =>Man's speed - Current speed = 12.5 - 2.5 = 10 km/hr.

Bishu Murmu said: (Jun 10, 2019)  
Thanks all for explaining this.

Praveen said: (Aug 1, 2019)  
MS with CS = 15.
CS = 2.5,
MS = 15-2.5= 12.5.

Against means Opp direction so it is Upstream For this MS - CS so 12.5 -2.5 = 10.

Nihal said: (Oct 2, 2019)  
A boat has speed along current of 5 km/h and speed against current is 1 km/h. What will be speed of boat and speed of Current? please give me the solution.

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