Consider the following premises. For all x_3, x_3 will take a plane if and only if x_3 is a server. There is at least one x_3 for which x_3 is a server if and only if x_3 will take a plane. Can we infer the following from them? If we can, name the inference rule being used: There is at least one x_3 for which if x_3 were a server, then x_3 is taking a plane.
 From the fact that there is at least one x_3 for which x_3 is a server if and only if x_3 will take a plane we can infer that there is at least one x_3 for which if x_3 takes a plane, then x_3 is a server via existential biconditional elimination. From the fact that for all x_3, x_3 will take a plane if and only if x_3 is a server we can infer that for all x_3, if x_3 were a server, then x_3 is taking a plane via universal biconditional elimination. From the fact that there is at least one x_3 for which if x_3 takes a plane, then x_3 is a server, and that for all x_3, if x_3 were a server, then x_3 is taking a plane we can infer that there is at least one x_3 for which x_3 will take a plane if and only if x_3 is a server via existential biconditional introduction. Finally, from the fact that there is at least one x_3 for which x_3 will take a plane if and only if x_3 is a server we can infer that there is at least one x_3 for which if x_3 were a server, then x_3 is taking a plane via existential biconditional elimination. Therefore, the answer is yes.