A boy uses a homemade metal detector to look for valuable metal objects on a beach. The machine isn't perfect-it identifies only
98% of the metal objects over which it passes, and it identifies 4% of the
nonmetallic objects over which it passes. Suppose that 25% of the objects
that the machine passes over are metal. Choose an object from this beach
at random. If the machine gives a signal when it passes over this object,
find the probability that the boy has found a metal object.