Correct option is : (a) 20 km/h
When the man is at rest with respect to the ground, the rain comes to him at an angle 30∘ with the vertical. This is the direction of the velocity of raindrops with respect to the ground. The situation when the man runs is shown in the figure
\(V_{rg}\)= velocity of rain drops with respect to ground.
\(V_{man}\)= velocity of man =10 km/h
Hence, \(\cos\ \theta = \frac{10}{V_{rg}}\)
\(\Rightarrow \cos 60^\circ = \frac{10}{v_{rg}}\)
\(\Rightarrow \frac{1}{2} =- \frac{10}{V_{rg}}\)
\(\Rightarrow V_{rg} = 20\ km/h\)
Hence, velocity of rain drops with respect to the ground is 20 km/h.
