Consider the following premises. David is taking a plane if and only if Mary is an office clerk. David is writing a letter if and only if Barbara is a taxi driver. For all x_20, if x_20 writes a letter, then x_20 is taking a plane. Can we infer the following from them? If we can, name the inference rule being used: The fact that Barbara were a taxi driver does not imply that Mary is an office clerk.
That contradicts the premises. Therefore the answer is no.