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

$A$ student performed the experiment of determination of focal length of a concave mirror by $u-v$ method using an optical bench of length $1.5 \ m$. The focal length of the mirror used is $24 \ cm$. The maximum error in the location of the image can be $0.2 \ cm$. The $5$ sets of $(u, v)$ values recorded by the student (in $cm$) are: $(42, 56), (48, 48), (60, 40), (66, 33), (78, 39)$. The data set$(s)$ that cannot come from the experiment and is (are) incorrectly recorded,is (are):
$(A) (42, 56)$
$(B) (48, 48)$
$(C) (66, 33)$
$(D) (78, 39)$

The image formed by a convex mirror of focal length $f$ is $(1/n)$ times the size of the object. The distance of the object from the mirror is:

$A$ square cardboard of side $3\, cm$ is placed $25\, cm$ from a concave mirror of focal length $10\, cm$. What is the area enclosed by the image of the cardboard? (The plane of the cardboard is perpendicular to the principal axis.)

An object is placed on the principal axis of a concave mirror at a distance of $1.5 f$ ($f$ is the focal length). The image will be at ....... $f$.

The relation between the linear magnification $m$,the object distance $u$,and the focal length $f$ is: