Consider the following premises. Thomas is playing squash if and only if David is listening to a song. If David listens to a song, then Richard is a sailor. If Richard were a sailor, then Thomas is playing squash. Can we infer the following from them? If we can, name the inference rule being used: Richard is a sailor if and only if David is listening to a song.
 From the fact that Thomas is playing squash if and only if David is listening to a song we can infer that if Thomas plays squash, then David is listening to a song via biconditional elimination. From the fact that if Richard were a sailor, then Thomas is playing squash, and that if Thomas plays squash, then David is listening to a song we can infer that if Richard were a sailor, then David is listening to a song via transitivity. Finally, from the fact that if Richard were a sailor, then David is listening to a song, and that if David listens to a song, then Richard is a sailor we can infer that Richard is a sailor if and only if David is listening to a song via biconditional introduction. Therefore, the answer is yes.