The compound interest received on ₹18,000 for 2 years is ₹7,920, when the interest is compounded annually. What is the rate of interest per annum?

$$compound\ interest\ =principal\left[\left(1+\frac{rate}{100}\right)^{time}-1\right]$$

$$7920=18000\left[\left(1+\frac{rate}{100}\right)^2-1\right]$$

$$\frac{11}{25}=\left[\left(1+\frac{rate}{100}\right)^2-1\right]$$

$$1+\frac{11}{25}=\left[\left(1+\frac{rate}{100}\right)^2\right]$$

$$\frac{25+11}{25}=\left[\left(1+\frac{rate}{100}\right)^2\right]$$

$$\frac{36}{25}=\left[\left(1+\frac{rate}{100}\right)^2\right]$$

$$\left(\frac{6}{5}\right)^2=\left(1+\frac{rate}{100}\right)^2$$

$$\frac{6}{5}=1+\frac{rate}{100}$$

$$\frac{6}{5}-1=\frac{rate}{100}$$

$$\frac{1}{5}=\frac{rate}{100}$$

rate of interest per annum = 20%