An aeroplane flies along four sides of a square field at speeds of 200,400,600 and 800, km / hr . av

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4.Average

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An aeroplane flies along the four sides of a square field at speeds of $200,400,600$ and $800\, km / hr$. The average speed of the plane in the flight around the field in $km / hr$ is:

A

$384$

B

$400$

C

$500$

D

$284$

Solution

Let the side of square be $a$ and average speed of aeroplane $x$

$\frac{a}{200}+\frac{a}{400}+\frac{a}{600}+\frac{a}{800}=\frac{4 a}{x}$

$\Rightarrow \frac{12 a+6 a+4 a+3 a}{2400}=\frac{4 a}{x} \Rightarrow \frac{25}{2400}=\frac{4}{x}$

$x=384$

Hence average speed of a aeroplane $=384 km / hr$

Standard 13

Quantitative Aptitude

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