I not understand

Dear Student,

Let the principal amount, and the rate of interest be P and R% per annum respectively.

Given : The sum of money doubles in 10 years

$$\begin{gathered} 2P = P +\frac{P\times R\times T}{100} \\ 2P-P =\frac{P\times R\times 10}{100} \\ P = \frac{PR}{10} \\ R = 10 ----\left(1\right) \end{gathered}$$

Now the money becomes 4 times in T years at the same rate of interest

$$\begin{gathered} 4P = P +\frac{P\times R\times T}{100} \\ 4P-P = \frac{P\times R\times T}{100} \\ 3P =\frac{P\times R\times T}{100} \\ 300P = P\times 10\times T ( From (1) R =10) \\ 300 =10T \\ T =30 years \end{gathered}$$

Option B

Regards