Question

A man can row at 6 km/h in still water and at 4 km/h upstream. how long will the man take to go to a place1 km downstream and return

Answer 1

Soln:

Given,

Velocity of man in still water = 6 km/h

Velocity of man in upstream water = 4 km/h

Time taken in roundtrip = ?

Since, velocity in upstream = velocity in still water - velocity of stream = 4 (Given)

or, Velocity of steam = 6 - 4 = 2 km/h

Hence, Velocity of man in downstream = velocity of man in still water + Velocity of stream

or, Velocity of man in downstream = 6 + 2 = 8 km/h

let t1 = time taken to go 1 km in downstream = 1/8 hour

t2 = time taken to return = 1/4 hour

Total time taken by man = (1/8 + 1/4) = 3/8 hour = 22.5 minutes = 1350 sec

Answer 2

speed for downstream = 6+4=10 kmph
Time taken to go 1 km downstream = 1/10 hrs =6 mins
speed for upstream = 6-4=2 kmph
Time taken to go 1 km upstream = 1/2 hrs =30 mins
$so \: total \: time = 6+30= 36 mins$