A hollow iron pipe is 21 cm long and its external diameter is 8 cm. If the thickness of the pipe is 1 cm and iron weighs 8 g/cm³, then the weight of the pipe is:

🧠 Explanation (Math):

Given: Pipe $L=21$ cm, $R=4$ cm, thickness 1 ⇒ $r=3$ cm, density $=8$ g/cm³
Step 1: Volume $=pi(R^2-r^2)L=pi(16-9)cdot21=147pi$ cm³.
Step 2: Mass $=147pitimes8approx3694.5$ g $=3.6945$ kg (≈3.696 kg).
Final Answer: 3.696 kg