Correct Answer - Option 3 : 20 hours
Given:
Speed in 1st half is 15 km/h
Speed in 2nd half is 5 km/h
Total distance covered is 150 km
Formula:
Average speed for a journey when distances are equal is S = 2{xy/(x + y)}
Where,
S =average velocity
x = speed during 1st part of journey
y = speed during 2nd part of journey
Calculation:
Average speed for the whole journey = 2 × {(15 × 5)/(15 + 5)} km/h
⇒ Average speed = 2 × (75/20) km/h
⇒ Average speed = (75/10) km/h
Time to cover the distance = 150/(75/10) hours
⇒ Time = (150 × 10)/75 hours
⇒ Time = 20 hours
∴ Time taken to cover the distance is 20 hours