An object is placed at a distance $u$ from an equiconvex lens such that the distance between the object and its real image is minimum. The focal length of the lens is $f$. The value of $u$ is

The graph shows part of variation of $v$ with change in $u$ for a concave mirror. Points plotted above the point $P$ on the curve are for values of $v$

The graph shows how the inverse of magnification $\frac{1}{m}$ produced by a convex thin lens varies with object distance $u$. What was the focal length of the lens used?