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~40s

Doubling in eight years

At what rate of simple interest will a sum double itself in 8 years?

Show the answer and the worked solution

Correct answer: B.

12.5%

Worked solution

  1. To double, SI must equal P, i.e. R × T = 100.
  2. 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: RT = 100; tripling: RT = 200.

Shortcut. 12.5% is exactly 1/8, so eight years of one-eighth each add one whole principal — the sum doubles.

Common trap. 10% comes from borrowing the compound-interest rule of 72 — 72 ÷ 8 = 9%, nearest option 10%. Under simple interest a sum doubles when R × T = 100, so R = 100 ÷ 8 = 12.5%.

Simple & Compound Interest

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Level

Easy

Target time

~40s

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Doubling in eight years — Aptitude Question

Doubling in eight years: At what rate of simple interest will a sum double itself in 8 years? Answer and worked solution on CodeKairo.

Question

At what rate of simple interest will a sum double itself in 8 years?

Options

  1. 10%
  2. 12.5%
  3. 15%
  4. 16%
Show the answer and the worked solution

Correct answer: B. 12.5%

Worked solution

  1. To double, SI must equal P, i.e. R × T = 100.
  2. 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: RT = 100; tripling: RT = 200.

Shortcut. 12.5% is exactly 1/8, so eight years of one-eighth each add one whole principal — the sum doubles.

Common trap. 10% comes from borrowing the compound-interest rule of 72 — 72 ÷ 8 = 9%, nearest option 10%. Under simple interest a sum doubles when R × T = 100, so R = 100 ÷ 8 = 12.5%.

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