Nine times under compound interest — Aptitude Question
Nine times under compound interest: A sum triples in 4 years at compound interest. In how many years will it become… Answer and worked solution on CodeKairo.
- Topic: Simple & Compound Interest
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 60 seconds
Question
A sum triples in 4 years at compound interest. In how many years will it become nine times itself?
Options
- 6
- 8
- 12
- 16
Show the answer and the worked solution
Correct answer: B. 8
Worked solution
- Nine times = 3², that is two successive triplings.
- Time = 2 × 4 = 8 years.
Answer: 8.
The idea
Express the target multiple as a power of the known multiple, then multiply the time by that exponent.
Shortcut. 1 → 3 takes 4 years and 3 → 9 takes the same 4 years, because each 4-year stretch multiplies by 3: 8 years.
Common trap. 12 — counting 9 ÷ 3 = 3 triplings instead of 9 = 3²; two triplings reach nine times.
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