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Answer :

Diameter of wooden sphere = 21 cm

Radius of wooden sphere, (r)

= \(\frac{diameter}{2} \)

= (\(\frac{21}{2} \)) cm

= 10.5 cm

Surface area of a wooden sphere

\( = 4\pi r^2 \)

\( = 4×(\frac{22}{7} )×(10.5)^2 = 1386 \)\( cm^2\)

Radius of the circular end of cylindrical support = 1.5 cm

Height of cylindrical support = 7 cm

Curved surface area of one cylindrical support

= 2\(\pi \)rh

= 2×(\(\frac{22}{7} \))×1.5×7

= 66 \( cm^2 \)

Now,

Area of the circular end of cylindrical support

= \( \pi r^2 \)

= \(\frac{22}{7} ×(1.5)^2 = 7.07 \)

Again,

Area to be painted silver

= [8 ×(1386-7.07)]

= 8×1378.93

= 11031.44

Area to be painted is 11031.44 \( cm^2 \)

Cost for painting with silver colour

= Rs(11031.44×0.25)

=Rs 2757.86

Area to be painted black

= (8×66) \( cm^2 \)

= 528 \( cm^2 \)

Cost for painting with black colour

=Rs (528×0.05)

= Rs26.40

Therefore, the total painting cost is:

= Rs(2757.86 +26.40)

= Rs 2784.26

- Height = 110cm, Depth = 25cm, Breadth = 85cm (see figure). 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 \(cm^2 \)and the rate of pointing is 10 paise per \(cm^2 \) find the total expenses required for palishing and painting the surface of the bookshelf.
- The diameter of a sphere is decreased by 25%. By what per cent does its curved surface area decrease?

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