At what rate per annum on simple interest will a sum of money double itself in 8 years?

Correct option is C

Given:

Amount becomes double of itself

Time = 8 years

Formula Used:

​$$SI = \frac{P\times R\times T}{100}$$ 

here, SI = Simple interest, P = principal, R = rate of interest, T = time

Amount = Principle + Simple Interest

Solution: 

Amount = 2 $$\times$$​ Principle

2P = P + SI

SI = P 

Now, from the formula;

​$$SI = \frac{P\times R\times T}{100}$$​​

​$$P = \frac{P\times R\times 8}{100}$$​​

​$$R = \frac{P\times 100}{P \times 8 }$$​​

​$$R = 12.5\%$$ 

Thus,  the rate of Interest, R is 12.5%.