physics
posted by Sonu .
A man running horizontal road with velocity 8 km/hr. Find the rain falling vertically downward direction. When he increases speed to 12 km/hr he find rain drops are falling at an angle 30' with downward vertically. What will be the velocity of rain?

I am interpreting your first two sentences as meaning:
"A man running horizontally along a road, with velocity 8 km/hr, finds the rain falling in a vertically downward direction."
That statement implies that the rain has a horizontal component of 8 km/hr in the same direction the runner is running. When he runs at the same speed, the rain apears to come straight down.
If he increases his speed to 12 km/hr, there is a 4 km/hr relative velocity of wind into his face. The vertical component of the wind velcity, Vy, must such that 4/Vy = tan 30
Vy = 6.93 km/hr
Vx = 8.00 km/hr
Wind speed (magnitude) = 10.58 km/hr 
could we keep the answer as 4root7?

yes