Consider the following premises. For all z_4, z_4 will take a plane if and only if z_4 will play squash. For all z_4, if z_4 plays squash, then z_4 is taking a plane. For all z_4, if z_4 takes a plane, then z_4 is playing squash. Can we infer the following from them? If we can, name the inference rule being used: For all z_4, z_4 will play squash if and only if z_4 will take a plane.
 From the fact that for all z_4, z_4 will take a plane if and only if z_4 will play squash we can infer that for all z_4, if z_4 takes a plane, then z_4 is playing squash via universal biconditional elimination. Finally, from the fact that for all z_4, if z_4 plays squash, then z_4 is taking a plane, and that for all z_4, if z_4 takes a plane, then z_4 is playing squash we can infer that for all z_4, z_4 will play squash if and only if z_4 will take a plane via universal biconditional introduction. Therefore, the answer is yes.