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Aman covers 1/4th of total distance at the speed of 36 km/hr, 1/3rd of total distance with 72 km/hr and remaining distance at 144 km/hr. What will be the average speed of whole journey of Aman?


1. 75 km/hr
2. 69.12 km/hr
3. 80 km/hr 
4. 84 km/hr

1 Answer

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Best answer
Correct Answer - Option 2 : 69.12 km/hr

Given:

Speed of Aman for 1/4th of total distance = 36 km/hr

Speed of Aman for 1/3rd of total distance = 72 km/hr

Formula:

Average Speed = Total Distance/Total time taken

Calculation:

Take LCM (3, 4) = 12

Let the total distance = 12x

⇒ 1/4th of total distance = ¼ × 12 = 3x

⇒ 1/3th of total distance = 1/3 × 12 = 4x

⇒ Remaining distance = 12x – (3x + 4x) = 5x

⇒ Average speed = 12x/[(3x/36) + (4x/72) + (5x/144)]

⇒ Average Speed = 12x/[(12x + 8x + 5x)/144] = (12 × 144)/25 = 69.12 km/hr

∴ The average speed of Aman is 69.12 km/hr 

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