Velocity of train, vT = 30 km /hr
Velocity of car, vc = 40 km/hr
To find: relative velocity of car w.r.t. train. To bring the train at rest, apply equal and opposite velocity of train on car, which is 30 km/hr.
So, relative velocity of car w.r.t train is,
vCT = OD = (OB2 + BD2)1/2
= (402 + 302)1/2
= 50 km/hr
Let <DOB = , then