How many cubic meters 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 ₹ 4.50 per sq meter.

Given,

Diameter of well = 2.8 m

Radius of the well = $\dfrac{2.8}{2}$ = 1.4 m

Depth of the well = 28 m.

Volume of earth dug out = Volume of well = πr²h

= $\dfrac{22}{7}$ x 1.4 x 1.4 x 28

= 172.48 m³.

Area of curved surface = 2πrh

= 2 x $\dfrac{22}{7}$ x 1.4 x 28

= 2 x 22 x 0.2 x 28

= 246.4 m².

Given, the cost of plastering at the rate of ₹ 4.50 per sq meter.

∴ Cost of plastering inner surface = ₹ 246.40 x 4.50

= ₹ 1108.80.

Hence, volume of earth to be dug out = 172.48 m³ and cost of plastering inner surface = ₹ 1108.80.