Each wheel of a car is of diameter 80 cm. How many complete revolutions does each wheel make in 10 minutes when the car is travelling at a speed of 66 km per hour ?

Diameter of wheel = 80 cm

Radius of the wheel = $\dfrac{d}{2}$ = $\dfrac{80}{2}$ = 40 cm

Circumference of the wheel = 2πr

$= 2 \times \dfrac{22}{7} \times 40\\[1em] = \dfrac{1,760}{7} \text{ cm}$

Let the wheel make n number of revolutions.

Speed of the car = 66 km per hour

Time = 10 min

Total distance = Speed x Time

= $\dfrac{66 \times 100,000}{60}$ x 10 cm

= 11 x 100,000 cm

= 1,100,000

Total distance = Number of revolutions x Circumference of the wheel

⇒ 1,100,000 cm = n x $\dfrac{1,760}{7}$ cm

⇒ n = $\dfrac{1,100,000 x 7}{1,760}$

⇒ n = $\dfrac{7,700,000 }{1,760}$

⇒ n = 4,375

Hence, the wheel makes 4,375 complete revolutions.