Beer coaster maths

It is associated with basic/rough figures traditionally done on the back of a beer coaster at a pub while drinking.

An example might be mates working out the cost of a fishing trip, so "Fuel will be about $100, bait about $50, accom about $200 and good about $200". So rough/approximate figures.

According to my beer coaster maths, it would be about $500. But if you want a more accurate number I'd obviously have to look it up, speak to the suppliers etc.