Discussion

What will be total cost of polishing curved surface of a wooden cylinder at rate of Rs. 20 per sq.m, if it has 40 cm diameter and 7m height?

Answer: D

Convert the diameter to metres before anything else, since the rate is per square metre.

\[ d = 40 \text{ cm} = 0.4 \text{ m} \quad\Longrightarrow\quad r = 0.2 \text{ m} \]

\[ \text{CSA} = 2\pi r h = 2 \times \frac{22}{7} \times 0.2 \times 7 = 8.8 \text{ m}^{2} \]

\[ \text{cost} = 8.8 \times 20 = \text{Rs. } 176 \]

Only the curved surface is polished, so the two circular ends are excluded; including them would add \( 2\pi r^{2} = 0.251 \) m².

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