A wheel of a car of radius 21 cms is rotating at 600 RPM. What is the speed of the car in km/hr? Choose the correct choice from the following answers:	A) 79.2 km/hr	B) 47.52 km/hr	C) 7.92 km/hr	D) 39.6 km/hr	E) 3.96 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 * 600= 
<gadget id="calculator">132 * 600</gadget>
<output>79_200</output>
79200 cm in a minute.   In an hour, the wheel will cover a distance of 79200 * 60= 
<gadget id="calculator">79200 * 60</gadget>
<output>4_752_000</output>
4752000 cm.   Therefore, the speed of the car= 4752000 cm/hr = 47.52 km/hr   Choice 47.52 km/hr is the correct answer.
<result>B</result>
A wheel of a car of radius 21 cms is rotating at 600 RPM. What is the speed of the car in km/hr?