A circular pond has a circumference of 22 km. What is its diameter?

Question

A circular pond has a circumference of 22 km. What is its diameter?

1. A. 15 km
2. B. 1.5 km
3. C. 0.7 km
4. D. 7 km (Correct answer)

Correct Answer

Option D — 7 km

Detailed Solution & Explanation

The correct answer is 7 km.

Key Points

• The circumference of a circle in terms of its diameter is:
  • $C = \pi d$
• Substituting $C = 22$ km and $\pi = \frac{22}{7}$:
  • $22 = \frac{22}{7} \times d$
  • $d = 22 \times \frac{7}{22} = 7 \text{ km}$

Shortcut Trick

• Whenever a circumference is given as a multiple of 22, the $\frac{22}{7}$ cancels instantly. Here $22 \div 22 = 1$, leaving $d = 7$ directly — no long division needed.

Additional Information

• Using $C = 2\pi r$ instead gives $r = 3.5$ km, so $d = 2r = 7$ km — the same result. Reading the question carefully matters, since 0.7 km and 1.5 km are placed as options precisely to catch a radius-for-diameter slip.
• Values worth memorising for $\pi = \frac{22}{7}$: a circumference of 22 gives $d = 7$; of 44 gives $d = 14$; of 66 gives $d = 21$.