Consider the following premises. There is at least one x_3 for which x_3 will take a plane. For all x_3, x_3 will take a plane if and only if x_3 is an office clerk. Can we infer the following from them? There is at least one x_3 for which x_3 is an office clerk.
 From the fact that for all x_3, x_3 will take a plane if and only if x_3 is an office clerk we can infer that for all x_3, if x_3 takes a plane, then x_3 is an office clerk. Finally, from the fact that for all x_3, if x_3 takes a plane, then x_3 is an office clerk, and that there is at least one x_3 for which x_3 will take a plane we can infer that there is at least one x_3 for which x_3 is an office clerk. Therefore, the answer is yes.