A man decided to cover 7.5 km in 72 minutes. He decided to cover two thirds of the distance at 5 km/h and the remaining at some different speed. Find the speed after two third distance has been covered.

Correct option is C

​Total distance = 7.5 km.
Total time = 72 minutes = 72/60 = 1.2 hours.
Distance to be covered at first part = (2/3) × 7.5 km = 5 km.
Time to cover the first part of the distance at 5 km/h:
Time = Distance / Speed =5 km / 5 km/h = 1 hour.
Remaining distance = 7.5 km - 5 km = 2.5 km.
Remaining time = Total time - Time_1 = 1.2 hours - 1 hour = 0.2 hours.
Required Speed = 2.5 km / 0.2 hours = 12.5 km/h.​