On a certain sum, the simple interest for 2 years is ₹1,800 and the compound interest (compounded annually) for 2 years at the same rate is ₹1,881. What is the sum?

Explanation

1. 01

   One year's SI = ₹900. The extra ₹81 is interest on that ₹900 for one year.

2. 02

   Rate = $\frac{81}{900} \times 100 = 9\%$.

3. 03

   Sum = $\frac{1800 \times 100}{9 \times 2} = 10000$, that is, ₹10,000.