The front compound wall of a house is decorated by wooden spheres of diameter 21cm, placed on small supports. Eight such spheres are used for this purpose, and are to be painted silver. Each support is a cylinder of radius 1.5 cm and height7 cm and is to be painted black. Find the cost of paint required if silver paint costs 25 paise per cm2 and black paint costs 5 paise per cm2
Radius (r) of wooden sphere = (
) = 10.5 cm
Surface area = 4 πr2
= 4 * 
* 10.5 * 10.5
= 1386 cm2
Radius of circular end (r1) = 1.5 cm
Height (h) = 7 cm
CSA = 2 πrh
= 2 * 
* 1.5 * 7
= 66 cm2
Area of the circular end of cylindrical support = πr2
= 
* 1.5 * 1.5
= 7.07 cm2
Area to be painted silver = [8 × (1386 − 7.07)]
= (8 × 1378.93)
= 11031.44 cm2
Cost of painting silver color = Rs (11031.44 × 0.25) = Rs 2757.86
Area to be painted black = (8 × 66)
= 528 cm2
Cost for painting with black color = Rs (528 × 0.05) = Rs 26.40
Total cost in painting = Rs (2757.86 + 26.40)
= Rs 2784.26