The wheel of a car has 210 cm diameter. How many revolutions per minute must the wheel make so that the speed of the car is kept at 120 km/h?

$$\eqalign{ & {\text{Diameter}} = 210{\text{ cm}} \cr & {\text{Radius}} = 105{\text{ cm}} \cr & {\text{Circumference}} = 2\pi r \cr & = 2 \times \frac{{22}}{7} \times 105 \cr & = 44 \times 15 \cr & = 660{\text{ cm}} \cr & {\text{1 hr}} \to {\text{120 km}} = 120000{\text{ m}} \cr & {\text{1 min}} \to \frac{{120000}}{{60}} = 2000{\text{ m}} \cr & {\text{Number of revolution}} = \frac{{2000 \times 100}}{{660}} = 303.03 \cr} $$