Prashant travelled from Aligarh to Delhi which are at a distance of 150 km. He travelled first 20 km at 40 kmph, next 75 km at 50 kmph and the remaining distance at 22.5 kmph. He returned by the same road but at a constant speed of 37.5 kmph. Find his average speed over the whole journey.?

Option 3 : 37.5 kmph

Given:

Distance between Aligarh and Delhi = 150 km

Speed for first 20 km = 40 kmph

Speed for next 75 km = 50 kmph

Speed for remaining distance = 22.5 kmph

Speed for return journey of 150 km = 37.5 kmph

Formula used:

Average speed = Total distance travelled/ Total time taken

Time = Distance/Speed

Calculation:

Time taken to travel 20 km = 20/40 = 1/2 hour = 30 min

Time taken to travel 75 km = 75/50 = 1 hour 30 min

⇒ Remaining distance = 150 – (20 + 75) = 55 km

Time taken to travel 55 km = 55/22.5 = 2 hour

Time taken to travel 150 km = 150/37.5 = 4 hour

Total distance travelled = 300 km

⇒ Total time taken = 8 hours

∴ Average speed = 300/8 = 37.5 kmph