Rs 10000 is kept at compound interest at an interest rate of 18% per annum (compounding annually). If the compounding of interest is done half yearly, then how much more interest (in Rs) will be obtained?

Principal sum = Rs. 10,000

Time period = 1 year and rate of interest = 18%

=> Difference between compound interest compound annually and half yearly = $$[P(1+\frac{R}{200})^{2T}-P]-[P(1+\frac{R}{100})^T-P]$$

= $$P[(1+\frac{18}{200})^2-(1+\frac{18}{100})^1$$]

= $$10,000[(1+\frac{9}{100})^2-(1+\frac{18}{100})]$$

= $$10,000[(\frac{109}{100})^2-(\frac{118}{100})]$$

= $$10,000[\frac{(109)^2-(118\times100)}{10000}]$$

= $$11881-11800=Rs.$$ $$81$$

=> Ans - (C)