A hollow cylindrical pipe is 21 dm long.

Let the length of the cylinder pipe be $h=21, \mathrm{dm}=210 \mathrm{~cm}$.

Let the outer and the inner radius of the pipe be $R \mathrm{~cm}$ and $r \mathrm{~cm}$, re $s$ pectively.

$\therefore 2 R=10$ and $2 r=6$

$R=5 \mathrm{~cm}$ and $r=3 \mathrm{~cm}$

Volume of the copper used in making the pipe, $V=\pi\left(R^{2}-r^{2}\right) h$

$=\frac{22}{7} \times\left(5^{2}-3^{2}\right) \times 210$

$=22 \times(25-9) \times 30$

$=22 \times 16 \times 30$

$=10560 \mathrm{~cm}^{3}$