If a pint is £2 and you spend £10 on five pints, how come if you have ten pints you go home having spent about £40?

because your pissed mate has asked you (who is pissed as well) for £20 until tomorrow, and you've also bought both of you kebabs and dropped him off home in your taxi (usually well out of your way) - oh and you told the taxi driver to keep the change from a tenner. Plus you dropped all the loose pound coins in the back of the taxi seat from your pocket (which the taxi driver collects at the end of the night)

You know me too well.