While performing an experiment for determining the focal length of...

While performing an experiment for determining the focal length of a concave mirror by u-v method, a student recorded the given sets of the positions (in cm) of the object and the corresponding image on the bench as $(12,51),(18,54),(30,50),(48,34),(42,42)$ and $(78,98)$. She used an optical bench of length 1.5 m and the mirror is fixed at the 90 cm mark on the bench. The maximum acceptable error in the location of the image is 0.2 cm . The reading (observation) that cannot be obtained from experimental measurement and has been incorrectly recorded, for a mirror of focal length $=24 \mathrm{~cm}$, is