Aptitude - Boats and Streams - Discussion

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.

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

Please explain me, why man's rate subtract form the current,

I'm not understanding.

I'm not understanding

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

You are right samarth.

Thank you. Now I understand clearly.

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

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.

Thanku balaji.