A wheel of a car of radius 21 cms is rotating at 800 RPM. What is the speed of the car in km/hr?  Choose the correct choice from the following:	A) 79.2 km/hr	B) 47.52 km/hr	C) 7.92 km/hr	D) 39.6 km/hr	E) 63.36 km/hr
Explanatory Answer   The radius of the wheel measures 21 cm.   In one rotation, the wheel will cover a distance which is equal to the circumference of the wheel.   ∴ in one rotation this wheel will cover 2 * π * 21 = 132 cm.   In a minute, the distance covered by the wheel = circumference of the wheel * rpm   ∴ this wheel will cover a distance of 132 * 800= 
<gadget id="calculator">132 * 800</gadget>
<output>105_600</output>
105600 cm in a minute.   In an hour, the wheel will cover a distance of 105600 * 60= 
<gadget id="calculator">105600 * 60</gadget>
<output>6_336_000</output>
6336000 cm.   Therefore, the speed of the car= 6336000 cm/hr = 63.36 km/hr   Choice 63.36 km/hr is the correct answer.
<result>E</result>
A wheel of a car of radius 21 cms is rotating at 800 RPM. What is the speed of the car in km/hr?