a room is 9 m long. 8m wide and 4m high. find the cost of plastering its wall and ceiling at the rate of RS 250 per square metre

We want to cover the walls and ceiling with plaster. Since we understand that the measure of the space "inside" a shape is called area, we know we are looking for the area of the four walls and the ceiling.  Each wall and the ceiling are rectangles, and the area of a rectangle is calculated by multiplying width times length:  a = lw

Two of the walls are 9m by 4m:  a = lw;  a = (9)(4);  a= 36 sq meters.

Two of the walls are 8m by 4m:  a = lw; a = (8)(4); a = 32 sq meters.

The ceiling measures 9m by 8m:  a = lw; a = (9)(8); a = 72 sq meters.

The total area we need to cover with plaster is   36 + 36 + 32 + 32 + 72 (all four walls and the ceiling).  Total area is 208 sq meters.

The cost is 250 per square meter, so we just need to multiply our total square meters by the cost per square meter.  

                    208 sq meters x Rs 250 = Rs 52,000.