1. We are given the principal amount $P=100000$, the simple interest $SI=25200$, and the time period $t=9$ years. We need to find the rate of interest $r$. 2. Recall the formula for simple interest: $$SI = \frac{P \times r \times t}{100}$$ 3. Substitute the given values into the formula: $$25200 = \frac{100000 \times r \times 9}{100}$$ 4. Simplify the fraction on the right side: $$25200 = 100000 \times r \times 0.09$$ $$25200 = 9000r$$ 5. Solve for $r$: $$r = \frac{25200}{9000} = 2.8$$ 6. The rate of interest used was $2.8\%$ per annum.