In an experiment to measure the focal length \( (f) \) of a convex lens, the magnitude of object distance \( (x) \) and the image distance \( (y) \) are measured with reference to the focal point of the lens. The \( y-x \) plot is shown in figure.
The focal length of the lens is \( cm \).
\( \begin{array}{llll}10 & 20 & 30 & 40\end{array} \)
