The shape of pencil = cylinder.
Let the radius of base = r cm
Circumference of base = 1.5 cm
Circumference of circle is 2πr = 1.5 cm
r = 1.5/2π cm
According to the question,
Height, h = 25 cm
We know that,
Curved surface area of cylinder = 2πrh
Curved surface area of pencil = 2π (1.5/2π) 25 = 37.5 cm2
1 cm = 0.1 dm
1 cm2 = 0.01 dm2
37.5 cm2 = 0.375 dm2
Cost for coloring 1 dm2 = Rs. 0.05
Cost for coloring 0.375 dm2 (i.e. 1 pencil) = Rs. 0.01875
Cost for coloring 120000 pencils = 120000 × 0.01875 = Rs. 2250