Aptitude - Boats and Streams - Discussion

@All.

Against the current - upstream.
Along the current - downstream.

If they mentioned the question like "the speed of the man is...", definitely the answer will be 12.5 km/hr, ie. upstream = speed of the man - speed of the current = 15-2.5=12.5 km/hr.

But,
The question was like "Speed of the man with the current".

ie., 15km/hr is the summation of the speed of the man and the current. so we need to subtract the speed of the current from 15, to know man's speed in water.
ie., 15 - 2.5 = 12.5 => man's speed.
After the above step, we can easily find the man's speed against the current by using the formula below:
upstream = speed of the man - speed of the current.
= 12.5 - 2.5 = 10 km/hr.

Not yet understand the solution. Please help me to get this.

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 @Balaji.

Thank you for explaining.

I am not understanding this. Please explain me.

Yes, you are right @Balaji.

Good Explanation, Thanks a lot.

Here, the given speed of the man 15 km/hr but not given directly speed of the man 15 km/hr then we subtract 15-2.5 then we get the speed of the man.

Hey guys.

If they said the man speed is ..the answer is 12.5...but;
The man speed with the current.
So the 15 km/hr have ( man speed + Current speed).
So the answer is 10km/hr.

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.

@All.

Suppose you are riding a boat which is arriving to the beach. Naturally, the waves are in the same direction of your boat. Dont you feel easy to drive it then. Hence, the speed of waves/current supplement your boat speed. Then, it is called going downstream or going with the waves/current. Hence, By thinking vice-versa we know that we'd have to subtract the speed of the stream