# GMAT Practice Question: Solid Geometry

Concept: Circumference of a circle, Speed of travel, Converting values from one unit to another.

This GMAT practice question is a quant problem solving question from Solid Geometry (Mensuration). Concept: computing circumference of a wheel (circle) and speed of the car. In quite a few questions in solid geometry, you will have to pay attention to the unit in which data is presented and convert units, if necessary, appropriately to arrive at the correct answer.

#### Question: A wheel of a car of radius 21 cms is rotating at 600 RPM. What is the speed of the car in km/hr?

1. 79.2 km/hr
2. 47.52 km/hr
3. 7.92 km/hr
4. 39.6 km/hr
5. 3.96 km/hr

Video explanation will be added soon

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 = 79200 cm in a minute.

In an hour, the wheel will cover a distance of 79200 * 60 = 4752000 cm.

Therefore, the speed of the car = 4752000 cm/hr = 47.52 km/hr

Choice B is the correct answer.

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