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Surface area of a sphere = 4πr²

Curved surface area of a cylinder = 2πrh

Base area of a cylinder = πr²

Given, front compound wall of a house is decorated by wooden spheres of diameter 21 cm placed on small supports.

Each support is a cylinder of a radius 1.5 cm and height 7 cm and is to be painted black

Thus, the surface area of the sphere would be reduced by the cylindrical supports base.

Total surface area of the spherers = 8 × (4 × (22/7) × (21/2)² – (22/7) × 1.5²) = 11031.43 cm²

Total curved surface area of the spheres = 8 × 2 × (22/7) × 1.5 × 7 = 528 cm²

Silver paint costs 25 paise per cm² and black paint cost 5 paise per cm²

Total cost of painting = 11031.43 × 0.25 + 528 × 0.05 = Rs. 2784.26