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