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Assuming the earth to be a sphere of radius 6370 km, how many square kilometres is area of the land, if three-fourth of the earth’s surface is covered by water?
Surface area of a spere = 4πr2
Given, earth is a sphere of radius 6370 km.
Surface area of earth = 4 × (22/7) × 63702
⇒ Surface area of earth = 510109600 km2
Now, three-fourth of the earth’s surface is covered by water
Area covered by land = 1/4 × 510109600 = 127527400 km2
The front compound wall of a house is decorated by wooden spheres of diameter 21 cm, placed on small supports as shown in Fig. 21.11. Eight such spheres are used for this purpose, and are to be painted silver. Each support is a cylinder of a radius 1.5 cm and height 7 cm and is to be painted black. Find the cost of paint required if silver paint costs 25 paise per cm2 and black paint cost 5 paise per cm2.