$\rho =\frac{ M }{\pi r ^2 \ell}$

$\frac{\Delta \rho}{\rho} =\frac{\Delta M }{ M }+\frac{2 \Delta r }{ r }+\frac{\Delta l}{l}$

$\frac{\Delta \rho}{\rho} \%$ $=\left[\frac{0.002}{0.4}+\frac{2(0.001)}{(0.3)}+\frac{0.02}{5}\right] \times 100 \%$

$=\frac{1}{2} \%+\frac{2}{3} \%+\frac{2}{5} \%$

$=1.6 \%$