A wheel of a car of radius 21 cms is rotating at 900 RPM. What is the speed of the car in km/hr?  Choose the correct choice: A) 71.28 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 * 900= 
<gadget id="calculator">132 * 900</gadget>
<output>118_800</output>
118800 cm in a minute.   In an hour, the wheel will cover a distance of 118800 * 60= 
<gadget id="calculator">118800 * 60</gadget>
<output>7_128_000</output>
7128000 cm.   Therefore, the speed of the car= 7128000 cm/hr = 71.28 km/hr   Choice 71.28 km/hr is the correct answer.
<result>A</result>
A wheel of a car of radius 21 cms is rotating at 900 RPM. What is the speed of the car in km/hr?