Consider the following premises. If it is overcast, then David is a janitor. If Robert goes running, then it is overcast. If David were a janitor, then Richard is taking a plane. It is not overcast. If Richard takes a plane, then Robert is running. Can we infer the following from them? Richard is taking a plane.
 From the fact that if Richard takes a plane, then Robert is running, and that if Robert goes running, then it is overcast we can infer that if Richard takes a plane, then it is overcast. From the fact that if it is overcast, then David is a janitor, and that if David were a janitor, then Richard is taking a plane we can infer that if it is overcast, then Richard is taking a plane. From the fact that if it is overcast, then Richard is taking a plane, and that if Richard takes a plane, then it is overcast we can infer that it is overcast if and only if Richard is taking a plane. Finally, from the fact that it is overcast if and only if Richard is taking a plane, and that it is not overcast we can infer that Richard is not taking a plane, which contradicts that Richard is taking a plane. Therefore, the answer is no.