Consider the following premises. For all x_12, x_12 will sleep and x_12 is a cashier. If Robert were a server, then David is not sleeping. Can we infer the following from them? If we can, name the inference rule being used: Robert is not a server.
 From the fact that for all x_12, x_12 will sleep and x_12 is a cashier we can infer that for all x_12, x_12 will sleep via universal simplification. From the fact that for all x_12, x_12 will sleep we can infer that David is sleeping via universal instantiation. Finally, from the fact that if Robert were a server, then David is not sleeping, and that David is sleeping we can infer that Robert is not a server via modus tollens. Therefore, the answer is yes.