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# The wheel of a railway carriage is 40cm in diameter and makes 6 revolutions in a second;how fast is the train going?

$\begin{array}{1 1}(A)\;20cm/sec\\(B)\;24\pi cm/sec\\(C)\;24cm/sec\\(D)\;24cm/sec\end{array}$

Toolbox:
• $r=\large\frac{d}{2}$
• Circumference =$2\pi r$
Given :
Diameter of the wheel =40cm
$\therefore$ Radius of the wheel =20cm
Circumference of the wheel =$2\pi r$
$\Rightarrow 2\pi \times 20$
$\Rightarrow 40\pi$cm
Number of revolutions made in 1 second =$6$
$\therefore$ Distance covered in 1 second=$40\pi\times 6$
$\Rightarrow 240\pi$cm
$\therefore$ Speed of the train =$24\pi$cm/sec
Hence (B) is the correct answer.