How do you (fairly) divide expenses when married couples are involved?
 

For example:
<ul>[*]3 or 4 people go on a road trip, 2 of the people are married to each other. Do you divide gas by the number of people? Or does the married couple count as 1?[*]Buying food/drinks for parties[*]etc[*]etc[/list]It's easy to think of the married couple as one financial unit, so in a way it's unfair to "double charge" them for things. On the other hand, there's two of them, so why should the singles subsidize them? But what about things where there is no (or very little) marginal cost of adding that extra person? Like a road trip. If only 4 people are going, and the married couple was going anyway, is it really fair to make the married couple pay for 1/2 the gas? What do you do! How do you decide these things? Is there a way?

well think of it this way

when you take a trip on a plane, does the provider charge 1 ticket or 2 for a married couple?