A train is out of control, and in its path there are five people who will be killed if it continues on that track. You can switch the train onto another track, but there's a single person who will die
if the train is diverted to that track.
In a Mills Utilitarian view: You would need to pull the switch to change the track to kill only the one person instead of the five. This is because saving the 5 people is providing more happiness for more people than if you only save the one person.