Al Reynolds wrote;

You will think that I am slightly sad for having noticed
this, but on pondering the 13.5% extra free offer on some
beer cans, I realised that:
440ml + 13.6% = 500ml (0.5 litre)
500ml + 13.6% = 568ml (1 pint)

I can't believe this is a coincidence.

Neither can I. But why 13:5% extra free? Another odd figure! Why not 10%
or 15%?

The plot thickens.................

Dave