Question

In a 600 km trip a mans driving speed was 50 km/h for two thirds of the trip if he completes the trip in 10 hours find his driving speed

Answer 1

the answer is 100 km/h

Answer 2

since it's not clear what you are asking about so I assume that you are asking speed to cover remaining one third distance in remaining time

two third trip = 400kms
time taken to travel 400kms = 400/50 = 8hrs     (time = distance/speed)
remaining time = 2hrs 
remaining distance = 200km
speed required to cover remaining distance in remaining time= 200/2 = 100km/h                           (speed = distance/time)
ANSWER = 100KM/H