this problem is
(a) quite hard (there's plenty of literature about it on the net - see the game theory article in the Wall Street Journal Number Guy's blog (2005))
(b) interesting in that solutions depend on social dispositions!
(c) not very well specified - is there a base cost for hiring the cab or not?
this version of the problem was used by Rob Tait (NCETM) at the Education Show (2014)
the simplest way to split the fare (dividing in the ratio of the mileages) may not be deemed to be the fairest...
a matter for debate
and some form of model (e.g. a bar picture) seems an appropriate response