Gothamist: The Economics of Splitting a Cab:

5. The Talmudic method: 'To solve the problem, you need to consider all three possible pairs of two riders from the group of three, imagine them haggling over the savings, and come up with an overall solution -- modeled after the Talmud's teaching on the divided estate -- that works for all three negotiations. The math was formalized in a Nobel Prize-winning paper by Robert Aumann and Michael Maschler 20 years

Wow...the math must be amazing...