Given that the focal length of the lens is f = 4.20 cm
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the maximum magnification is observed when the image formed by the lens is at near point of
the lens which is corresponds to q = -25 cm
then the focal length is
1/f = 1/ p + 1/ q
1/ 4.20cm = 1/ p + 1/ (-25cm)
then the object distance is p = --------- cm
the magnification is m = - q / p = - (-25cm) / (----cm)
= ------