#### Question

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

#### Solution

We have,

Speed of the car = 66 km/hr

∴ Distance travelled by the car in 1 hour = 66 km

⇒ Distance travelled by the car in 10 min.

`= \frac { 66 }{ 60 } × 10 km = 11 km = 11 × 1000 × 100 cm`

We have,

Radius of car wheels = 40 cm

∴ Circumference of the wheels

`= 2 × \frac { 22 }{ 7 } × 40 cm`

⇒ Distance travelled by the car when its wheels take one complete revolution

`= 2 × \frac { 22 }{ 7 } × 40 cm`

∴ Number of revolutions made by the wheels in 10 minutes

`=`

`=\frac{11\times 100\times 100}{2\times \frac{22}{7}\times40}=\frac{11\times 1000\times 100\times 7}{2\times 22\times 40}=4375`

Hence, each wheel makes 4375 revolutions in 10 minutes.