\(\rho =\frac{ M }{\pi r ^2 \ell}\)
\(\frac{\Delta \rho}{\rho} =\frac{\Delta M }{ M }+\frac{2 \Delta r }{ r }+\frac{\Delta l}{l}\)
\(\frac{\Delta \rho}{\rho} \%\) \(=\left[\frac{0.002}{0.4}+\frac{2(0.001)}{(0.3)}+\frac{0.02}{5}\right] \times 100 \%\)
\(=\frac{1}{2} \%+\frac{2}{3} \%+\frac{2}{5} \%\)
\(=1.6 \%\)