A cast-iron pipe is 1 m long with an inner (bore) diameter of 3 cm and uniform metal thickness of 1 cm. If cast iron has density 21 g/cm^3, find the pipe’s weight.

Introduction / Context:
The pipe is a hollow cylinder; its metal volume is the difference between the outer and inner cylinder volumes. Multiplying this volume by the material density gives the weight (mass) of the pipe.

Given Data / Assumptions:

• Length L = 1 m = 100 cm.
• Inner diameter = 3 cm ⇒ inner radius r_i = 1.5 cm.
• Thickness t = 1 cm ⇒ outer radius r_o = 2.5 cm.
• Density ρ = 21 g/cm^3.

Concept / Approach:

• Metal volume V = π*(r_o^2 − r_i^2)*L (in cm^3).
• Weight (mass) m = ρ * V.

Step-by-Step Solution:

Verification / Alternative check:
Using π ≈ 3.1416 gives 400*3.1416*21 ≈ 26389 g; rounding to one decimal kg yields 26.4 kg.

Why Other Options Are Wrong:

• 21, 24.2, 18.6 kg: Do not match the precise hollow-cylinder calculation.
• None of these: Not applicable; 26.4 kg is correct.

Common Pitfalls:

• Using diameter instead of radius.
• Forgetting to convert cm to m or grams to kilograms.

Final Answer:
26.4 kg