Aptitude - Boats and Streams - Discussion

The upstream and downstream speeds are calculated on relative motion.

If two bodies are moving in the same direction their speed is substracted and if they're moving in an opposite direction they are added.

Guys, it's very simple you don't need to find upstream or downstream speed you can the total time like this.

Let T be total time and t1=return time and t2=going time and D = be the distance(105) x=still water speed of man and y=stream speed so,

T = t1+t2
T = (D/x+y)+(D/x-y) {definitely one direction will be upstream n oppsite dir will b downstream}
T = D{(2x)/(x^2 - y^2).

Solution.
T = (105 *2*9)/(9^2-1.5^2).
T = (1890)/(78.75),
T = 24 Hours.

How? explain this.

How do you arrive at upstream and down stream speed.

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.