If 1 cubic cm of cast iron weights 21 gm, then the weight of a cast iron pipe of length 1 m with a bore of 3 cm and in which the thickness of the metal is 1 cm, is?

External radius = 2.5 cm,
length = 100 cm
∴ External volume = [π x (2.25)² x 100] cm²
internal radius = 1.5 cm
∴ internal volume = [π x (1.5)² x 100]cm³
Volume of metal
= [π x (2.5)² x 100 - π x (1.5)² x 100] cm³
= π x 100 x [(2.5)² - (1.5)²] cm³
= (22/7 x 100 x 4 x 1 x 21/1000) kg
= 26.4 kg.