Consider the following premises. For all x_8, x_8 will take a plane if and only if x_8 will make tea. For all x_8, x_8 will not make tea. There is at least one x_8 for which x_8 will climb a mountain if and only if x_8 will take a plane. Can we infer the following from them? There is at least one x_8 for which x_8 will not climb a mountain.
 From the fact that there is at least one x_8 for which x_8 will climb a mountain if and only if x_8 will take a plane we can infer that there is at least one x_8 for which if x_8 climbs a mountain, then x_8 is taking a plane. From the fact that for all x_8, x_8 will take a plane if and only if x_8 will make tea, and that for all x_8, x_8 will not make tea we can infer that for all x_8, x_8 will not take a plane. Finally, from the fact that there is at least one x_8 for which if x_8 climbs a mountain, then x_8 is taking a plane, and that for all x_8, x_8 will not take a plane we can infer that there is at least one x_8 for which x_8 will not climb a mountain. Therefore, the answer is yes.