A ring-shaped disc has outer radius 10 cm and inner radius 7 cm. What is the approximate ratio of the ring's area to the whole ou…

Question

A ring-shaped disc has outer radius 10 cm and inner radius 7 cm. What is the approximate ratio of the ring's area to the whole outer circle?

1. A. 1:2 (Correct answer)
2. B. 2:3
3. C. 3:4
4. D. 4:5

Correct Answer

Option A — 1:2

Detailed Solution & Explanation

The correct answer is 1:2.

Key Points

• Ring (annulus) area = π(10² − 7²) = π(100 − 49) = 51π.
• The whole outer circle = π × 10² = 100π.
• Ratio = 51π : 100π ≈ 0.51 ≈ 1 : 2.
• An annulus is the difference of two circles (outer² − inner²); compare it to the outer circle by cancelling π and rounding.