Consider the following premises. Susan is a janitor if and only if David is an office clerk. It is sunny if and only if Susan is a janitor. If it is sunny, then David is an office clerk. Can we infer the following from them? If we can, name the inference rule being used: If it is sunny, then Susan is a janitor.
 From the fact that Susan is a janitor if and only if David is an office clerk we can infer that if David were an office clerk, then Susan is a janitor via biconditional elimination. Finally, from the fact that if it is sunny, then David is an office clerk, and that if David were an office clerk, then Susan is a janitor we can infer that if it is sunny, then Susan is a janitor via transitivity. Therefore, the answer is yes.