Physics - Error Question with Solution | TestHub

On an optical bench, a $10\mathrm{cm}$ needle calibrates the holders. Initial readings are $x_O=1.1\mathrm{cm}$, $x_I=21.0\mathrm{cm}$ and $x_L=10.9\mathrm{cm}$. Find the correction to add to the image-side measured separation, in cm.