Aptitude - Boats and Streams - Discussion

How there 41/2 will came?

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.

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.

How can calculate distance of upstream ?

How can calculate distance of upstream ?

Friends this step is little confused any can help please?

(30/(15+v))+(30/(15-v)) = 4 1/2.

(30*30/(15-v)(15+v)) = 4 1/2.

Speed in still water = 15 km/hr.
Distance traveled in downstream = 30 km.

Time taken in upstream = 4 hr 30 min.
The speed of the stream = ?

Speed = Distance/time.
Distance = 30km.

Speed in upstream = (u-v)km/hr = (15-v)km/hr.

Speed in downstream = (u+v)km/hr = (15+v)km/hr.

Total time = (distance of downstream/speed of downstream) + (distance of upstream/speed of upstream).

(30/(15+v))+(30/(15-v)) = 4 1/2.

(30*30/(15-v)(15+v)) = 4 1/2.

900/(15^2 - v^2) = 4 1/2.

900/(225 - v^2) = 4 1/2.

900 / (225 - v^2) = 9/2.

900 * 2 = 9(225 -v^2).

1800 = 2025 - 9v^2.

9v^2 = 2025 - 1800.

9v^2 = 225.

v^2 = 225/9.

v^2 = 25.

v = 5 Km/hr.

Thank You...!

Seetha

Very well Explained Seetha ...Thank you.....

By solving the fraction 4*2=8, n 8+1=9, so the ans is 9/2