Consider the following premises. Richard is sad or David is not working. If Sarah takes a plane, then David is working. If Richard were sad, then Sarah is taking a plane. Can we infer the following from them? Richard is sad if and only if David is working.
 From the fact that Richard is sad or David is not working we can infer that if David works, then Richard is sad. From the fact that if Richard were sad, then Sarah is taking a plane, and that if Sarah takes a plane, then David is working we can infer that if Richard were sad, then David is working. Finally, from the fact that if Richard were sad, then David is working, and that if David works, then Richard is sad we can infer that Richard is sad if and only if David is working. Therefore, the answer is yes.