A motor-cycle covers 50 km with a speed of 25 km/hr. What must be the speed of the motor-cycle for the next 50 km journey so that the average speed of the whole journey is 30 km/hr?

A

$$25\frac{1}{2}$$ km/hr

B

$$35\frac{1}{2}$$ km/hr

C

$$37\frac{1}{2}$$ km/hr

D

$$38\frac{1}{2}$$ km/hr