A personal observation.

I regularly come accross 1966 coins in my pockets - the date decimal currency was introduced in Australia, even the odd predecimal 6 pence (the 5c, 10c and 20c decimal coins were based on the 6 pence, 1 shilling and 2 shilling pre decimal versions, which were themselves based on their English equivalent. 

I suspect the calculaion would need to be based on analysis of a large sample of coins, which would indicate a gradual dimminishment of older coins after a certain age.

Gary Vines
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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