Consider the following premises. There is at least one x_9 for which x_9 will not take a plane. For all x_9, x_9 is a scientist. For all x_9, x_9 is a scientist if and only if x_9 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_9 for which x_9 will make tea.
 From the fact that for all x_9, x_9 is a scientist we can infer that for all x_9, x_9 is a scientist or x_9 will make tea via universal addition. From the fact that for all x_9, x_9 is a scientist if and only if x_9 will take a plane, and that there is at least one x_9 for which x_9 will not take a plane we can infer that there is at least one x_9 for which x_9 is not a scientist via existential biconditional elimination. Finally, from the fact that for all x_9, x_9 is a scientist or x_9 will make tea, and that there is at least one x_9 for which x_9 is not a scientist we can infer that there is at least one x_9 for which x_9 will make tea via existential disjunctive syllogism. Therefore, the answer is yes. Notice, however, that the premises were contradictory, so we can infer anything from them.