Ratio of circumferences — Mensuration Aptitude Question
Ratio of circumferences: Two circles have radii in the ratio 3 : 5. What is the ratio of their circumferences? Answer and worked solution on CodeKairo.
- Topic: Mensuration
- Section: Quantitative Aptitude
- Difficulty: Easy
- Time target: 45 seconds
Question
Two circles have radii in the ratio 3 : 5. What is the ratio of their circumferences?
Options
- 3 : 5
- 9 : 25
- 5 : 3
- 27 : 125
Show the answer and the worked solution
Correct answer: A. 3 : 5
Worked solution
- Circumference = 2πr, so it scales directly with r.
- The ratio stays 3 : 5.
Answer: 3 : 5.
The idea
Circumference scales with the first power of r, area with the square and volume with the cube. Knowing which applies is the whole question.
Shortcut. Every length of a circle — radius, diameter, circumference — keeps the radius ratio, so 3 : 5 carries straight over.
Common trap. "9 : 25" — squaring the ratio as for area; circumference grows with r, not r².