Aptitude - Boats and Streams - Discussion

Thanks balaji.

Here they did not mention that is/clearly not questioned that this is a boad and stream problem.

Thanku balaji.

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.

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

Thank you. Now I understand clearly.

You are right samarth.

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

I'm not understanding

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

I'm not understanding.