$g=\frac{1}{4 \pi^{2}} \frac{T^{2}}{\ell}$

$\frac{\Delta g}{g}=\frac{2 \Delta T }{ T }+\frac{\Delta \ell}{\ell}$

$\frac{\Delta g}{g}=2 \cdot \frac{1}{100 \times 0.5}+\frac{1\,mm }{10\,cm }$

$\frac{\Delta g}{g}=\frac{5}{100}$