Doubling under simple interest — Aptitude Question
Doubling under simple interest: At what rate of simple interest does a sum double itself in 8 years? Answer and worked solution on CodeKairo.
- Topic: Simple & Compound Interest
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 50 seconds
Question
At what rate of simple interest does a sum double itself in 8 years?
Options
- 10%
- 12%
- 12.5%
- 15%
Show the answer and the worked solution
Correct answer: C. 12.5%
Worked solution
- To double, SI must equal P, so R × T = 100.
- R = 100 ÷ 8 = 12.5%.
Answer: 12.5%.
The idea
Under simple interest a sum becomes n times itself when R × T = 100(n − 1). Doubling gives 100, tripling 200.
Shortcut. Doubling means the interest equals P, so each of the 8 years must earn 1/8 of P — and 1/8 = 12.5%.
Common trap. 12% — truncating 100 ÷ 8 to a whole number; 12% for 8 years is only 96%, just short of doubling.
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