From doubling to quadrupling — Aptitude Question
From doubling to quadrupling: A sum doubles itself in 10 years at simple 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 itself in 10 years at simple interest. In how many years will it become four times itself?
Options
- 20
- 25
- 30
- 40
Show the answer and the worked solution
Correct answer: C. 30
Worked solution
- In 10 years the interest earned equals P.
- To become 4P the interest must be 3P, which takes 3 × 10 = 30 years.
Answer: 30.
The idea
Under simple interest the interest grows linearly, so the time scales with the multiple minus one. Doubling in 10 means tripling in 20 and quadrupling in 30.
Shortcut. Doubling in 10 years fixes the rate at 10% a year; four times needs 300% interest, and 300 ÷ 10 = 30 years.
Common trap. 20 — doubling the doubling time as if the interest compounded (2 → 4 in another 10 years). Under simple interest 20 years only triples the sum.
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