we know time=distance/speed
First half, time is d/V1
second half, time is d/V2
total time is (d/V1) + (d/V2) = d((1/V1) + (1/V2)) = d((V1+V2) / (V1.V2))
total time = d((V1+V2) / (V1.V2))
total distance = 2d
Average velocity = 2d/ d((V1+V2) / (V1.V2))
= 2 V1.V2 / (V1+V2)
Like if satisfied