Mass = Density * Volume of the hollow cylinder
= 8 * pi * h * (r22 - r12)
= 8 * (22/7) * 21 (42 - 32) [ As outer diameter = 8 cm so radius = 4 cm and as thickness of pipe = 1 cm so inner radius = 3 cm ]
= 8 * 22 * 3 * 7
= 3.696 kg