\(\mathrm{g}=\frac{4 \pi^2 \ell}{\mathrm{T}^2}\)
\(\frac{\Delta \mathrm{g}}{\mathrm{g}}=\frac{\Delta \ell}{\ell}+\frac{2 \Delta \mathrm{T}}{\mathrm{T}}\)
\(\quad=\frac{0.2}{20}+2\left(\frac{1}{40}\right)\)
\(=\frac{0.3}{20}\)
\(\text { Percentage change }=\frac{0.3}{20} \times 100=6 \%\)