The difference between simple interest and compound interest on a certain sum of money in three years at 10% per annum rate is ₹15.50. The sum is

₹500

We know that CI for 3 years at the rate of 10% = 33.1%

and SI for 3 years for the same rate = 30%

So the difference = 33.1 - 30 = 3.1%

So, according to the question,

3.1% of principal = 15.50

Then, 100% of principal =  ₹500