Correct Answer - Option 3 : 20 hours

**Given:**

Speed in 1^{st} half is 15 km/h

Speed in 2^{nd} 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 1^{st} part of journey

y = speed during 2^{nd} 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**