Hello there!

Greetings!

It's quite easy,

We have ,velocity of the man in w.r.t. to river = v

velocity of the flow = u

Velocity of the man downstream = v + u

Velocity of the man downstream = v - u

Let the distance is d

Then

Time taken to travel the distance d downstream = d/(v+u)

time taken to travel the distance d ustreapm = d/(v-u)

Total time =d/(v+u) + d/(v-u) = 2vd/v^2-u^2

Now, Average speed = Total Distance/Total Time

= 2d/ (2vd/v^2-u^2)

=( v^2-u^2)/v

Thankyou