What is the interest rate per annum, if a sum of money invested at compound interest amount to Rs. 2400 in 3 years and in 4 years to Rs. 2,520?

Let 'R%' be the rate of interest

From the given data,

$\frac{2520}{2400} = \frac{1 + \frac{R}{100}}{4}\Rightarrow 1 + \frac{R}{100} = \frac{63}{60}\Rightarrow R = 5$

Hence, the rate of interest R = 5% per annum.