Aptitude - Boats and Streams - Discussion

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.

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.

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.

hi ! @Vijaya.

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

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.

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.

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.

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.

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.

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.