Solution:
Write the dimensions of roomSince, length
Since, breadth
Since, height
Now we write the formula of surface area of walls and floor of the room
Let the total surface area of the cuboidal room be
.Then we can write the formula of total surface area of room as
Since we have to find the surface area of four walls and roof of the room, we subtract the area of the floor. Let the surface area of four walls and roof of the room be denoted by
.Area of floor of the room
⇒Surface area of four walls and roof of the room
Calculate the value in RHS
Calculate the area to be white washed
Substitute the values of
in equation (1)⇒Area of walls to be white washed
⇒Area of walls to be white washed 
Add the terms within the bracket
⇒Area of walls to be white washed
Now we calculate the cost of white washing the total area obtained
Use a unitary method to calculate the cost of white washing the walls and roof by multiplying the total surface area of walls and walls by cost of white washing per unit square meter.
Cost of white washing 1 sq.m of room = Rs15
⇒Cost of white washing 85.5 sq.m of room
⇒Cost of white washing 85.5 sq.m of room 
∴ Cost of white washing four walls and roof of the room is
.Hence, option 1 is the correct Ans:.
