1 Answers

Option 2 : 60 km/hr

Given:

Average speed = 20 km/hr

Rides for 20 minutes at a speed of 15 km/hr

Rides for 30 minutes at a speed of 10 km/hr

Concept Used:

Speed = Distance/Time

Average speed = Total distance travelled/Total time taken

Calculation:

According to the first condition,

⇒ 15 km/hr = Distance/ 20 minutes

⇒ 15 = Distance/ (20/60) hours

⇒ 15 = Distance/(1/3)

⇒ Distance = 5 km

According to the second condition,

⇒ 10 km/hr = Distance/30 minutes

⇒ 10 = Distance/(30/60) hours

⇒ 10 = Distance/(1/2) hours

⇒ Distance = 5 km

Now in the last phase he drives his scooter for 10 minutes.

⇒ Total time = 20 minutes + 30 minutes + 10 minutes

⇒ Total time = 60 minutes

⇒ Total time = 1 hour

Let the distance covered by him in the last phase be x

⇒ Total distance covered = 5 + 5 + x

⇒ Total distance covered = 10 + x

Average speed = Total Distance covered/Total time taken 

⇒ 20 = (10 + x)/ 1 hour

⇒ 20 = 10 + x

⇒ x = 20 – 10

⇒ x = 10 

⇒ The distance covered by him in the last phase is 10 km.

⇒ Speed in the last phase = 10 km/10 minutes

⇒ Speed in the last phase = 10 km/(10/60)hours

⇒ Speed in the last phase = 10 × 6

⇒ Speed in the last phase = 60 km/hr

∴ His speed in the last phase is 60 km/hr.

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