Aptitude - Boats and Streams - Discussion

Discussion Forum : Boats and Streams - General Questions (Q.No. 5)
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:
3 km/hr
5 km/hr
8 km/hr
9 km/hr
Explanation:

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

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

Discussion:
38 comments Page 4 of 4.

Brinda Iyer said:   1 decade ago
But if in the same question distance traveled is asked, then what is the formula?

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.

Saloni nautiyal said:   1 decade ago
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 IS NOT IN THE OPTIONS

Ajit Jha(iit kharagpur) said:   1 decade ago
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

Debapriya dandapat said:   1 decade ago
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.

Manjit singh said:   1 decade ago
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?

Vivek patidar said:   1 decade ago
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.