At what rate per cent will a sum of money double itself in 30 years?

Correct option is C

Given: ​

Sum of money double itself in 30 years.

Solution: 

$$\text{Let the sum be } x \\P = x, \, SI = x, \, t = 30 \, \text{years} \\\text{As we know,} \\SI = \frac{P \times r \times t}{100} \\x = \frac{x \times r \times 30}{100} \\r = \frac{100}{30} \\r = 3 \frac{1}{3}\%$$​​