The state of pencil = chamber.
Let the sweep of base = r cm
Outline of base = 1.5 cm
Boundary of circle is 2πr = 1.5 cm
r = 1.5/2π cm
As per the inquiry,
Tallness, h = 25 cm
We realize that,
Bended surface space of chamber = 2πrh
Bended surface space of pencil \[=\text{ }2\pi \text{ }\left( 1.5/2\pi \right)\text{ }25\text{ }=\text{ }37.5\text{ }cm2\]
1 cm = 0.1 dm
1 cm2 = 0.01 dm2
37.5 cm2 = 0.375 dm2
Cost for shading 1 dm2 = Rs. 0.05
Cost for shading 0.375 dm2 (for example 1 pencil) = Rs. 0.01875
Cost for shading 120000 pencils = 120000 ×0.01875 = Rs. 2250