Distance of the object u = −25cm (negative because the object is in front of the lens), Maximum distance the person can see clearly, v = −100cm (also negative for the same reason)

Using lense formula, \(\frac{1}{f}\) = \(\frac{1}{v}\) - \(\frac{1}{u}\)

\(\frac{1}{f}\) = \(\frac{1}{-100}\) - \(\frac{1}{-25}\)

\(\frac{1}{f}\) = \(\frac{1}{-100}\) + \(\frac{1}{25}\) = \(\frac{- 1 + 4}{100}\)

\(\frac{1}{f}\) = \(\frac{3}{100}\) 

f = \(\frac{100}{3}\) = 33.33cm

Thus, the person needs a convex lens (since it is used to help a far-sighted person) with a focal length of approximately equal to 33.33cm