1. **State the problem:** We need to find the rate of simple interest given the principal amount, the amount after 4 years, and the time period. 2. **Identify the known values:** - Principal $P = 2500$ - Amount $A = 3500$ - Time $t = 4$ years 3. **Recall the formula for simple interest:** $$ A = P + SI $$ where $SI$ is the simple interest. 4. **Calculate the simple interest:** $$ SI = A - P = 3500 - 2500 = 1000 $$ 5. **Use the simple interest formula:** $$ SI = \frac{P \times R \times t}{100} $$ where $R$ is the rate of interest per annum. 6. **Substitute the known values and solve for $R$:** $$ 1000 = \frac{2500 \times R \times 4}{100} $$ 7. **Simplify the equation:** $$ 1000 = 100R $$ 8. **Solve for $R$:** $$ R = \frac{1000}{100} = 10 $$ **Final answer:** The rate of interest charged is $10\%$ per annum.