Physics Question from JEE Main 2023 · 30 January, Shift 1

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