Length of a room `=20ft` Breadth of a room `=15ft` Height of a room `=12ft` Area of the 4 walls of the room =Lateral surface area of the room `=2(l+b)h` `=2(20+15)12` `=24(35)=840sq.ft` The cost of painting the walls of the room at` ₹20` per sq.ft `=840xx₹ 20 = ₹ 16800`