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4.Average
medium
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