Simple vs Compound Interest Sum Calculation

Simple interest per year = $\frac{800}{2} = 400$. Difference between CI and SI for 2 years = $900 - 800 = 100$. This ₹100 is the interest on ₹400 for 1 year. Rate $r = \frac{100}{400} \times 100 = 25\%$. Since $SI = \frac{P \times r \times t}{100} \Rightarrow 800 = \frac{P \times 25 \times 2}{100} \Rightarrow P = 1600$.