Consider the following premises. For all x, x is not sad. For all x, x is sad if and only if x is a server. William is a server or Charles is an artist. Can we infer the following from them? If we can, name the inference rule being used: Charles is an artist.
 From the fact that for all x, x is sad if and only if x is a server, and that for all x, x is not sad we can infer that for all x, x is not a server via universal biconditional elimination. From the fact that for all x, x is not a server we can infer that William is not a server via universal instantiation. Finally, from the fact that William is a server or Charles is an artist, and that William is not a server we can infer that Charles is an artist via disjunctive syllogism. Therefore, the answer is yes.