Acar travels 50 kilometers west and 10 minutes. after reaching the destination the car troubles back to the starting point, again taking 5 minutes. what is the average velocity of the car

SPEED is

               (distance covered) / (time to cover the distance) .

The car's average SPEED is

                            (100 km)  /  (15min)

                      =     6-2/3 km per minute.

VELOCITY is  

             (distance and direction between start-point and end-point)
divided by
             (time to travel from start-point to end-point).

The car ended up at the starting point, so the distance between
stating-point and end-point is zero.

      Velocity  =  (zero) / (time to travel from start-point to end-point)

                    =  Zero .

The correct answer is  "c".  it is "c" because melting is an endothermic change, and endothermic get's energy from surroundings.