Consider the following premises. For all x_12, x_12 will work. It is late if and only if Jessica is climbing a mountain. If David were an office clerk, then it is late. For all x_12, if x_12 climbs a mountain, then x_12 is not working. Can we infer the following from them? David is an office clerk.
 From the fact that for all x_12, if x_12 climbs a mountain, then x_12 is not working, and that for all x_12, x_12 will work we can infer that for all x_12, x_12 will not climb a mountain. From the fact that for all x_12, x_12 will not climb a mountain we can infer that Jessica is not climbing a mountain. From the fact that it is late if and only if Jessica is climbing a mountain, and that Jessica is not climbing a mountain we can infer that it is not late. Finally, from the fact that if David were an office clerk, then it is late, and that it is not late we can infer that David is not an office clerk, which contradicts that David is an office clerk. Therefore, the answer is no.