A wheel of a car of radius 21 cms is rotating at 400 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) 31.68 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 * 400= 
<gadget id="calculator">132 * 400</gadget>
<output>52_800</output>
52800 cm in a minute.   In an hour, the wheel will cover a distance of 52600 * 60= 3168000 cm.   Therefore, the speed of the car = 3168000 cm/hr = 31.68 km/hr   Choice 31.68 km/hr is the correct answer.
<result>D</result>
A wheel of a car of radius 21 cms is rotating at 400 RPM. What is the speed of the car in km/hr?