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