Description for Correct answer:
Given, P = RS. 4000, n = 9 months = \( \Large \frac{3}{4}yr \)

and CI= RS. 630.

Amount = P + CI = 4000 + 630.50 = rs. 4630.50

According to the formula,

Amount = \( \Large P \left(1+ \frac{R}{100\times 4}\right)^{4n} \)

=> 4630.50=\( \Large 4000 \left(1+ \frac{R}{400}\right)^{4\times 3/4} \)

=> 4630.50=\( \Large 4000 \left(\frac{400+R}{400}\right)^{3} \)

=> \( \Large \frac{4630.50}{4000} \)=\( \Large \left(\frac{400+R}{400}\right)^{3} \)

=> \( \Large \frac{9261}{8000} \)=\( \Large \left(\frac{400+R}{400}\right)^{3} \)

=> \( \Large \left(\frac{21}{20}\right)^{3} \)=\( \Large \left(\frac{400+R}{400}\right)^{3} \)

=> \( \Large \frac{400+R}{400} \)=\( \Large \frac{21}{20} \)

=> 400+R=\( \Large 21\times 20 \)=420

R=420-400=20%