If a sum of money doubles itself at compound interest in $12$ years, in how many years will it become four times?

Under compound interest, a sum grows exponentially over equal intervals of time.
Let the initial principal be $P$.

* In $12$ years, it becomes $2P$ (doubles).
* In the next $12$ years, this amount $2P$ will double again, becoming $2\times 2P=4P$ (four times).

Mathematically, if a sum becomes $2$ times in $12$ years, it will become $2^n$ times in $n\times 12$ years.
We want it to become $4$ times, and since $4=2^2$, we have $n=2$.

$$\text{Total time}=2\times 12=24\text{ years}$$