A sum of Rs 4000 becomes Rs 7000 in 6 years at simple interest. If the rate of interest becomes double of itself, then what will be the amount (in Rs) after 6 years?

Principal sum = Rs. 4000 and time period = 6 years

=> Amount after simple interest = Rs. 7000

Thus, simple interest = Rs. (7000-4000) = Rs. 3000

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

=> Simple interest = $$\frac{P\times R\times T}{100}$$

=> $$\frac{4000\times r\times6}{100}=3000$$

=> $$240r=3000$$

=> $$r=\frac{3000}{240}=12.5\%$$

Now, if rate of interest becomes double of itself, => $$r'=12.5\times2=25\%$$

$$\therefore$$ Amount after 6 years = $$P+\frac{P\times R\times T}{100}$$

= $$4000+(\frac{4000\times25\times6}{100})$$

= $$4000+6000=Rs.$$ $$10,000$$

=> Ans - (D)