A tyre has outer radius 12 cm and inner radius 8 cm. What is the approximate ratio of the tyre's area to the whole outer circle?
Show answer & explanation
Correct answer: A
Explanation
The correct answer is option 1 i.e. 5 : 9.
Area of tyre = π × (R2 - r2)
Outer radius R = 12, inner radius r = 8
Area of tyre = 22/7 × (12 × 12 - 8 × 8)
= 22/7 × 80
= 1760/7
Area of full outer circle = 22/7 × 12 × 12
= 22/7 × 144
= 3168/7
Ratio of tyre area to outer circle area = (1760/7) ÷ (3168/7) = 5 : 9