Aptitude - Boats and Streams - Discussion

A little confusion in simplification. How 900 came? Explain.

Still water = 15 kmph
Distance = 30 km
The time taken = 4hrs 30min= 4.5hrs.
Time = distance/speed.
t = 30/15+x +30/15-x = 4.5
Guess if x = 5 then.
15+5 = 20kmph,
15-5 = 10kmph,
Since, 30/20 + 30/10 = 1.5 + 3 = 4.5hrs.
So, x = 5.

Thank You @Padmagadi.

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.

Hai, Mr. Rajesh Kumar, can you explain me in brief?

How it is possible to multiply the speed of upstream and downstream?

Explain briefly.

Hello friends,

In this Speed of the boat in still water is given that is 15Km/s. and here we find speed of stream. take the answer from bottom suppose i take 10 from bottom.

You can see that speed of boat in upstream become15-10=5 and distance of upstream is 30 given. Here you can find out the time in case of upstream time=dist/speed 30/5=6.

Similarly speed in case of downstream 15+10=25. Distance 30 given time you can find that is=30/25=6/5. In question, we have given total time 4hr 30min. We can see that the answer 10 is choosen by me is wrong. Take 5 in place of 10 you get the answer.

I don't really understand this question. Some one should please help me out.

Simply follow formula when only distance the, time t and speed of boat x given.. and asked for speed of stream.... use in formula,

d^2/(x^2-y^2)=t ,

9/2=(225-y^2)/15, ==> y^2=25 , y=5