1.

A man has built a cubical water tank in his house. All sides of it are equal to 1.8m. All sides except the base are to be covered by tiles having equal sides of 30cm. How much money will he spend on tiles if cost of tiles is ₹400/dozen?(a) ₹7000(b) ₹7200(c) ₹6000(d) ₹6500This question was addressed to me during an interview for a job.I want to ask this question from Surface Area of a Cuboid and a Cube in section Surface Areas and Volumes of Mathematics – Class 9

Answer»

The CORRECT option is (b) ₹7200

To explain I would say: Total surface area of the cubical tank = 6l^2

= 6(1.8)^2

= 19.44 m^2

= 194400 cm^2

It is given that tiles are to be covered leaving the base.

Hence, the total area that is to be covered by tiles = 19.44 – (1.8*1.8)

= 16.2 m^2

= 162000 cm^2

Area of ONE TILE = 30*30 = 900 cm^2

Hence, total number of tiles required = \(\frac{Total \,area \,that \,is \,to \,be \,covered \,by \,tiles}{Area \,of \,one \,tile}\)

= \(\frac{162000}{900}\)

= 180

180 tiles = \(\frac{180}{12}\) = 15 dozens of tiles

Total cost of tiles = RATE of tiles/dozen * Total number of dozens tiles required

= 400*18

= ₹7200.


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