Aptitude - Boats and Streams - Discussion

How can calculate distance of upstream ?

How can calculate distance of upstream ?

The distance in both down&upstream is 30 as given;.

Here we have to find the speed. So distance/speed=time.

(30/15+x) + (30/15-x) = 4 1/2 = 9/2(2X4+1/2 = 9/2).

By solving it we get the answer.

Actually all of them are wrong who are saying that the answer would be 5.

Because the first step itself is wrong.

If a man covers a certain distance at x km/hr and returns at y km/hr then the avg speed is 2xy/(x+y). I think you all know this.

So the first step will be,

2(15+x)(15-x)/(225-x2) = (30)/(4.5).

Guys think about it.

How there 41/2 will came?

Simple to find speed of stream to check option.

A) 4.
= 15+4/2 + 15-4/2.
= 4.25 hr so wrong.

B) 5.
= 15+5/2 + 15-5/2.
= 4.5 hr so right answer.

What is the difference between 4 sum and 8 sum having the same description?

How we will getting 30 sir?

See its simple to understand.

Given distance is 30km. Let assume speed of river is x.

Speed of boat is 15 once it goes downstream. So speed would be 15+x.

And upstream is 15-x.

Now what it told from going to 30 km time required would be 30/downstream and coming back speed would be 30/upstream.

So total time given here is 4.30 minutes which is 4*1/2 (1/2 here means 30 minutes which is half of 1 hr).

So adding both the times downstream and upstream would give total time which is 4*1/2.

i.e why its gives 30/(downstream speed)+30/(upstream) = 4*1/2.

And now you are smart enough to solve equations.

What confuses me is that I expected the time gained going down to be same as that lost going up.

Finding the velocities etc easy way to solve this, but I still find the thought difficult to get around that whatever the stream gives in time going down should be lost going back But its just not true!