A wooden bookshelf has external dimensions as follows : Height = 110 cm, Depth = 25 cm, Breadth = 85 cm (See Fig. 18.5). The thickness of the plank is 5 cm everywhere. The external faces are to be polished and the inner faces are to be painted. If the rate of polishing is 20 paise per cm2 and the rate of painting is 10 paise per cm2. Find the total expenses required for polishing and painting the surface of the bookshelf.
Given,
External dimensions of wooden shelf = 85cm×25cm×110cm
Thickness of plank = 5 cm
So,
area to be polished = area of four walls +area of back+ area of front beading
= [2(110+85)×25 +110×85 +(75×5)×4] = 21700 cm2
Cost of polishing at rate 20paise per cm2 = 
Internal surface area = area of five faces of 3 cuboids each of dimension (75cm×30cm×20cm)
Total surface area of 3 cuboids = 3[2(75×30+30×20+20×75)] – 3(75×30)
= 6(2250+600+1500) – 6750 = 19350 cm2
∴ cost of painting at rate 10p per cm2 = 
Hence, total expense = Rs.(4340+1935) = Rs. 6275