A plane flies along the four sides of a square field at

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A plane flies along the four sides of a square field at a speed of 200, 400, 600 and 800 km/hr. Then find the average speed of plane around the square field.

1. 394
2. 400
3. 414
4. 384

Correct Option: D

Let the side of the square field be x and the average speed of plane be y

x/200 + x/400 + x/600 + x/800 = 4x/y
⇒ 25x/2400 = 4x/y
⇒ y =384

∴ Average speed is 384 km/hr