Doubling under compound interest — Aptitude Question
Doubling under compound interest: A sum doubles in 5 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 doubles in 5 years at compound interest. In how many years will it become eight times itself?
Options
- 10
- 15
- 20
- 25
Show the answer and the worked solution
Correct answer: B. 15
Worked solution
- Eight times = 2³, that is three successive doublings.
- Each doubling takes 5 years, so 3 × 5 = 15 years.
Answer: 15.
The idea
Under compound interest the multiples are powers, not multiples: doubling in t years gives 4× in 2t and 8× in 3t. Under simple interest the answer would be 35 years.
Shortcut. Chain the doublings — 1 → 2 by year 5, 2 → 4 by year 10, 4 → 8 by year 15 — instead of scaling the multiple.
Common trap. 20 — counting 8 ÷ 2 = 4 doublings; 8 is 2³, so three doublings of 5 years each.
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