Lol, I understood them all. Perhaps it is because I do Maths and Physics at the moment, and want to do Engineering at University .

The first one is pretty simple Vox, so I'll try to explain. Since it is stated that there are an infinite number of mathematicians, we can assume that they will all order 1/2 of what the previous mathematician ordered.

The first one orders a beer. The second orders half a beer. The third, a quarter of a beer.
The first one ordered a whole beer, so we can ignore him. The ones after him, however, are an example of exponential decay. By this, I mean that the amount of beer ordered would get less and less. So, if the numbers we plotted on a graph, the curve would get closer and closer to the x-axis, but NEVER touch. Therefore, the infinite number of mathematicians would never actually order 2 whole beers, only 1.99999999999999999999999999 etc.

And so the barman just gives them 2, because mathematicians are silly.