Ans is B.
it took 10 years to double, will definitely take another 10 years to double again (currently we have 2k, and want another 2k) at the same rate.
Mathematical Deduction for the same is as follows:
Amount after a recurrent period of time ‘t’ in years will be : $P(1+\frac{r}{100})^{t}$
Lets find out time it taken to double the amount as an function of P and r :
$P(1+\frac{r}{100})^{t} = 2P$
$\Rightarrow (1+\frac{r}{100})^{t} = 2$
$\Rightarrow t\times \ln (1+\frac{r}{100}) = \ln2$
$\Rightarrow t = \frac{\ln2 }{\ln (1+\frac{r}{100})}$
you can conclude t is independent of P (Principle amount).
Hence for the same Rate, to double the amount required time duration doesn’t change.
We can even conclude that for any multiple of P required time duration is same.
