Question : A hemispherical bowl has a 3.5 cm radius. It is to be painted inside as well as outside. The cost of painting it at the rate of Rs. 5 per 10 sq. cm will be:
Option 1: Rs. 77
Option 2: Rs. 100
Option 3: Rs. 175
Option 4: Rs. 50
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Correct Answer: Rs. 77
Solution :
Given: Radius ($r$) = 3.5 cm
Outer surface area of hemisphere = $2\pi r^2$
Inner surface area of hemisphere = $2\pi r^2$ (here width is negligible)
Surface area of both sides = $2\pi r^2+2\pi r^2 = 4\pi r^2$
= $4×\frac{22}{7}×3.5×3.5$
= 154 cm$^2$
Total cost of painting at Rs. 5 per 10 cm = 154 × $\frac{5}{10}$ = Rs. 77
Hence, the correct answer is Rs. 77.
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