A man takes `3 h` to cover a certain distance along the flow and takes `6h` to cover the same distance opposite to flow. In how much time, he will cross this distance in still water.

Options

A. `3.5 h`
B. 4 h
C. `4.5 h`
D. 5 h

Answer

B

Explanation

`u : upsilon_(rg)` (velocity of river w.r.t. ground)
`u : upsilon_(mr)` (velocity of man w.r.t. river)
Along the flow, `upsilon_(mg)=upsilon+u,3=(d)/(upsilon+u)` …(i)
where `upsilon_(mg)` is velocity of man w.r.t. ground
Opposite to the flow, `upsilon_(mg)=upsilon-u, 6=(d)/(upsilon-u)` ....(ii)