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Question

# How many cubic metres of earth must be dug out to make a well 28 m deep and 2.8 m in diameter? Also, find the cost of plastering its inner surface at Rs.4.50 per sq.m.

A
142.48 m3 ; Rs. 1108.80
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B
153.48 m3 ; Rs. 1008.80
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C
172.48 m3 ; Rs. 1108.80
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D
112.48 m3 ; Rs. 1008.80
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Solution

## The correct option is C 172.48 m3 ; Rs. 1108.80Given, Earth has to dug out to make well 2.8 m in diameter and 28 m in height. Also cost of plastering the inner wall is 4.50 per sq. m Radius =Diameter2=1.4 mWell is cylindrical in shape.So, Volume of well to be dug out =πr2h⇒V=227×1.42×28⇒V=172.48 m3To find cost of plastering of we have to first find curved surface area of inner wall.Curved surface area =2πrh⇒CSA=2×227×1.4×28⇒CSA=246.4 m2Cost of plastering = Curved Surface Area× Rate per sq. meterCost =246.4×Rs. 4.50⇒ Cost =Rs.1108.8Hence, option C.

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