Mean length:

${\ell }_{\text{mean}}=\frac{20.00+19.75+17.01+18.25}{4}=\frac{75.01}{4}=18.75\text{cm}$

Absolute deviations from the mean:

$|\Delta {\ell }_1|=1.25$, $|\Delta {\ell }_2|=1.00$, $|\Delta {\ell }_3|=1.74$, $|\Delta {\ell }_4|=0.50$.

Mean absolute deviation:

$\Delta {\ell }_{\text{mean}}=\frac{1.25+1.00+1.74+0.50}{4}=\frac{4.49}{4}\approx 1.12\text{cm}$

Relative error:

$\frac{\Delta {\ell }_{\text{mean}}}{{\ell }_{\text{mean}}}=\frac{1.12}{18.75}\approx 0.06$