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An aeroplane flies along the four sides of a square field at the speeds of 200, 400, 600 and 800 km/hr. Find the average speed of the plane around the field.

Answer:

Explanation:

Let each side of the square field be $x$ km and let the average speed of the plane around the field be $y$ km/hr.

Then, $\frac{x}{200}+\frac{x}{400}+\frac{x}{600}+\frac{x}{800}$ $=\frac{4x}{y}$

$\Leftrightarrow y=\left(\frac{2400\times4}{25}\right)$ $=384$

Average speed = 384 km/hr.