Christopher is travelling by car to a city 100 km away. For the trip, he wants to average 50 km/h. However, owing to mechanical problems, he finds that when he has travelled halfway his average speed is only 25 km/h. Which one of the following best describes the required speed for the second half of the trip for Christopher’s average speed of 50 km/h to be achieved?
A 75 km/h
B 100 km/h
C 150 km/h
D It is not possible.

I know the answer is D but could someone please explain why? Thanks very much!

Question

Christopher is travelling by car to a city 100 km away. For the trip, he wants to average 50 km/h. However, owing to mechanical problems, he finds that when he has travelled halfway his average speed is only 25 km/h. Which one of the following best describes the required speed for the second half of the trip for Christopher’s average speed of 50 km/h to be achieved?
A	75 km/h 
B      100 km/h 
C      150 km/h 
D	It is not possible.

I know the answer is D but could someone please explain why? Thanks very much!

vincent-lyon.fr · Answer

Work out total distance over total time (definition of average speed).
Then find the time in which he has to travel the second half of his journey.