Consider the following premises. If David takes a plane, then Mary is bored. For all x_10, if x_10 were a sailor, then x_10 is taking a plane. If Mary were bored, then David is a sailor. Can we infer the following from them? If is not true that David is a sailor if and only if David is taking a plane.
 From the fact that for all x_10, if x_10 were a sailor, then x_10 is taking a plane we can infer that if David were a sailor, then David is taking a plane. From the fact that if David takes a plane, then Mary is bored, and that if Mary were bored, then David is a sailor we can infer that if David takes a plane, then David is a sailor. Finally, from the fact that if David were a sailor, then David is taking a plane, and that if David takes a plane, then David is a sailor we can infer that David is a sailor if and only if David is taking a plane, which contradicts that If is not true that David is a sailor if and only if David is taking a plane. Therefore, the answer is no.