A sum of Rs 4000 becomes Rs 6000 in 1 year at a certain rate of compound interest. What will be the sum (in Rs) after 4 years?

Principal sum = Rs. 4000 and time period = 1 year

Let rate of interest = $$r\%$$

=> Amount after compound interest = Rs. 6000

=> $$P(1+\frac{R}{100})^T=6000$$

=> $$4000(1+\frac{r}{100})^1=6000$$

=> $$1+\frac{r}{100}=\frac{6000}{4000}=1.5$$

=> $$\frac{r}{100}=1.5-1=0.5$$

=> $$r=0.5\times100=50\%$$

$$\therefore$$ Amount after 4 years = $$4000(1+\frac{50}{100})^4$$

= $$4000(1+\frac{1}{2})^4=4000(\frac{3}{2})^4$$

= $$4000\times\frac{81}{16}$$

= $$250\times81=Rs.$$ $$20,250$$

=> Ans - (C)