A sum of Rs. 12,000 deposited at compound interest becomes double after 5 years. After 20 years, it will become

As we know $$P(1+ \frac{r}{100})^t$$ is amount of compound interest where r is rate, P is principal amount and t is time.
So $$12000(1+ \frac{r}{100})^5 = 2 \times 12000$$
or $$(1+ \frac{r}{100}) = 2^(\frac{1}{5})$$      eq(1)
Now after 20 years compound interest will be = $$12000(1+ \frac{r}{100})^20$$        
or     $$12000 (2^{(\frac{1}{5})})^{20}$$         (from eq.(1))
or     $$ 12000 \times 16 = 192000$$