Imagine there’s a spacecraft that’s been damaged in deep space. There’s four survivors, three are badly wounded and one is relatively unharmed. There’s enough air for four humans to survive one day or one human to survive four days. The closest rescue ship is three days away. After assessing the situation and verifying the air supply, the three wounded crewmembers sacrifice themselves so the one is rescued.
Not exactly like that… :-)
http://en.wikipedia.org/wiki/R_v_Dudley_and_Stephens