Geometrical Optics

In an experiment for estimating the value of focal length of converging mirror, image of an object placed at \(40 \mathrm{~cm}\) from the pole of the mirror is formed at distance \(120 \mathrm{~cm}\) from the pole of the mirror. These distances are measured with a modified scale in which there are 20 small divisions in
\(1 \mathrm{~cm}\). The value of error in measurement of focal length of the mirror is \(\frac{1}{\mathrm{~K}} \mathrm{~cm}\). The value of \(\mathrm{K}\) is __________.