A wheel of a car of radius 21 cms is rotating at 500 RPM. What is the speed of the car in km/hr?
Choose the correct choice from the following choices:
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 * 500= 
<gadget id="calculator">132 * 500</gadget>
<output>66_000</output>
66000 cm in a minute.   In an hour, the wheel will cover a distance of 66000 * 60= 
<gadget id="calculator">66000 * 60</gadget>
<output>3_960_000</output>
3960000 cm.   Therefore, the speed of the car= 3960000 cm/hr = 39.6 km/hr   Choice 39.6 km/hr is the correct answer.
<result>D</result>
A wheel of a car of radius 21 cms is rotating at 500 RPM. What is the speed of the car in km/hr?