When a person is at rest with respect to ground, the rain is coming to him at an angle 30o with the vertical, i.e., along OB.
As the person moves along OA with velocity 10 m/s, the relative velocity of rain w.r.t person is along OC as shown in fig. below.
Therefore, velocity of rain w.r.t ground =
Velocity of person w.r.t. ground =
Velocity of rain w.r.t person =
= 10/ (1/) = 10
Therefore, velocity of train w.r.t person = 10 m/s