Consider the following premises. For all x_4, if x_4 drinks water, then x_4 is taking a plane. For all x_4, x_4 will take a plane if and only if x_4 will drink water. There is at least one x_4 for which if x_4 takes a plane, then x_4 is drinking water. Can we infer the following from them? If we can, name the inference rule being used: There is at least one x_4 for which x_4 will drink water if and only if x_4 will take a plane.
We can infer this via the existential biconditional introduction rule. Therefore, the answer is yes.