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:

3.6 kg
3.696 kg
36 kg
36.9 kg

Correct answer: 3.696 kg

Solution

The pipe is a hollow cylinder. Outer radius = 4 cm and inner radius = 3 cm, length = 21 cm. Volume of iron = C0(4² - 3²)×21 = C0×7×21 = 462 cm³ using C0 = 22/7. Multiplying by density 8 g/cm³ gives 3696 g = 3.696 kg.

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