Aptitude - Boats and Streams - Discussion

How? explain this.

How to know downstream and upstream?

Speed of boat in still water = 9 kmph.
Speed of stream = 1.5 kmph.

Given distance from boat to a place is 105 km.

Consider a man want to go to that place, means he want go in opposite direction of boat( away from boat).

So we take upstream = sp(boat)-sp(stream) = 9-1.5 => 7.5kmph.
Time taken to travel in upstream is 105/7.5 = 14 hours.

By returning he travel towards boat sp(boat)+sp(stream) = 9+1.5 =>10.5kmph.
Time taken to travel in downstream is 105/10.5 = 10 hours

Total time taken to travel == 14+10 = 24 hours.

On what basis Upstream and down stream speed is calculated?

Speed of boat in still water is 9km how could you consider it as upstream speed similarly to downstream also they couldn't mention anything like that. Please explain.

9 km/hr is the speed of boat in standing water.

Boat has to move twice the distance of 105km to reach at same point.

Then (105/9)+(105/9) = 23.33333333333333333.

Then It should be 24hr.

I have a doubt, 9 km in question is speed of Boat, and a man rows 105 km but his speed is not given, and we have to find time taken by man.

Just calculate downstream. 10 hours. Upstream has to be more than 10. So only 24 possible.

Average speed During the Journey = (2*(9+1.5)*(9-1.5))/(9+1.5)+
(9-1.5)

so,Average speed=8.75 kmph

so,Total time= total distance/average speed
=(105*2)/8.75
= 24 hours.

Time taken for up stream = 105/(9-1.5)=14h
Time taken for down stream =
105/(9+1.5)=10h Total time taken= 14+10=24h