A certain sum becomes 3-fold at 4% annual rate of interest. At what rate, will it become 6-fold?

10%

The correct answer is Option (2) → 10%

Assuming simple interest:

• 3-fold amount ⇒ Interest = 3P - P = 2P

Given rate = 4% p.a.

$P \times \frac{4}{100} \times t = 2P \Rightarrow t = \frac{2}{0.04} = 50 \text{ years}$

Now, for the same time (50 years), to become 6-fold:

• Interest = 6P - P = 5P

$P \times \frac{r}{100} \times 50 = 5P \Rightarrow r = \frac{5}{50} \times 100 = 10\%$