Consider the following premises. Joseph is not working. For all x_3, if x_3 takes a plane, then x_3 is working. For all x_3, if x_3 works, then x_3 is taking a plane. Can we infer the following from them? If we can, name the inference rule being used: Joseph is taking a plane.
 From the fact that for all x_3, if x_3 works, then x_3 is taking a plane, and that for all x_3, if x_3 takes a plane, then x_3 is working we can infer that for all x_3, x_3 will work if and only if x_3 will take a plane via universal biconditional introduction. From the fact that for all x_3, x_3 will work if and only if x_3 will take a plane we can infer that Joseph is working if and only if Joseph is taking a plane via universal instantiation. Finally, from the fact that Joseph is working if and only if Joseph is taking a plane, and that Joseph is not working we can infer that Joseph is not taking a plane via biconditional elimination, which contradicts that Joseph is taking a plane. Therefore, the answer is no.