1. To correct hyperopia (far sightedness), convex lenses are prescribed. If a senior citizen can read a newspaper only when he holds it no closer than 50 cm away, what focal length of lens should be prescribed so he can read when he holds the book 25 cm away?
2. To correct myopia (nearsightedness), concave lenses are prescribed. If a student can read her physics book only when she holds it no further than 18 cm away, what focal length of lens should be prescribed so she can read when she holds the book 35 cm away?