We have,
Length of cylinder = 21 dm = 210 cm
Outer diameter = 10 cm
Outer radius, R = 10/2 = 5cm
Inner diameter = 6 cm
Inner radius, r = 6/2 = 3cm
∴ Volume of copper used in making the pipe = π (R2 – r2)h
= 22/7 (52 – 32) 210
= 22/7 (25-9) 210
= 10560 cm3