Q. 13.7( 99 Votes )

A wooden bookshelf has external dimensions as follows: Height = 110 cm, Depth = 25 cm, Breadth = 85 cm. 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.

Answer :

External height (l) of book self = 85 cm

External breadth (b) of book self = 25 cm


External height (h) of book self = 110 cm


The external surface area of the shelf while leaving out the front face of the shelf = lh + 2 (lb + bh)


= [85 × 110 + 2 (85 × 25 + 25 × 110)]


= (9350 + 9750)


= 19100 cm2


we know each stripe on the front surface is also to be polished. which is 5 cm stretch.

Area of front face = [85 × 110 − 75 × 100 + 2 (75 × 5)]            


= 1850 + 750


= 2600 cm2


Area to be polished = (19100 + 2600) = 21700 cm2


Cost of polishing 1 cm2 area = Rs 0.20


Cost of polishing 21700 cm2 area Rs (21700 × 0.20) = Rs 4340


It can be observed that length (l), breadth (b), and height (h) of each row of the bookshelf is 75 cm, 20 cm, and 30 cm respectively


Area to be painted in 1 row = 2 (l + h) b + lh


= [2 (75 + 30) × 20 + 75 × 30]


= (4200 + 2250)


= 6450 cm2


Area to be painted in 3 rows = (3 × 6450)


= 19350 cm2


Cost of painting 1 cm2 area = Rs 0.10


Cost of painting 19350 cm2 area = Rs (19350 × 0.1)


= Rs 1935


Total expense required for polishing and painting = Rs (4340 + 1935)


= Rs 6275

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