A classroom is 9 m long, 6 m wide and 4 m high…

Step 1. The walls. $2(a + b)c = 2 \times (9\ \text{m} + 6\ \text{m}) \times 4\ \text{m} = 120\ \text{m}^2$.

Step 2. The ceiling. $9\ \text{m} \times 6\ \text{m} = 54\ \text{m}^2$.

Step 3. The unpainted part. The door and two windows are $2\ \text{m}^2 + 2\ \text{m}^2 = 4\ \text{m}^2$.

Step 4. The area to paint. $120\ \text{m}^2 + 54\ \text{m}^2 - 4\ \text{m}^2 = 170\ \text{m}^2$.

Step 5. The cost. $170 \times 25 = 4250$ rupees.