Answer
$f=-0.50 \mathrm{~m}$
Work Step by Step
The far point of the person is $50 \mathrm{~cm}=0.50 \mathrm{~m}$ from the eye. The object distance is taken to be at infinity and the corrected lens will allow the image to be formed at the near the point. Thus,
$$
\frac{1}{f}=\frac{1}{p}+\frac{1}{i}=\frac{1}{\infty}+\frac{1}{-0.50 \mathrm{~m}}
$$
and we find the focal length of the lens to $f=-0.50 \mathrm{~m}$.
