UPSC Civil Services 2020 Prelims General Studies Paper II

Let certain distance =d
Let a man takes time in rowing a certain distance upstream =t
and downstream =‌

t

2

u= the speed of a man in still water
and v= the speed of the current
Then, ‌‌u+v=‌

d

t 2

⇒‌‌u+v=‌

2d

t

‌‌‌‌‌⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅(i)
⇒‌‌u−v=‌

d

t

‌‌‌‌‌⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅(ii)
From (i) and (ii), we get and
2u=‌

2d

t

+‌

d

t

=‌

3d

t

2v=‌

2d

t

−‌

d

t

=‌

d

t

∴
‌

2u

2v

=‌

3d∕t

d∕t

=‌

3

1

⇒‌‌u:v=3:1
Hence, the ratio of the speed in still water to the speed of current
=u:v=3:1.