Fewer than ten per cent of A$100 polymer banknotes ever issued have been returned to the RBA as unfit banknotes, the Reserve Bank of Australia said on Friday.It is already well known that "$100 banknotes are generally held by the public as a store of value [and] they do not tend to actively circulate," the RBA said in its quarterly Bulletin.Lower denomination banknotes on the whole do not last long enough to qualify for analysis on their durability.The $20 notes wear out fastest, lasting 4.6 years based on recent sampling, the RBA said.The $10 notes last 4.9 years and the $5 notes 5.2 years  - while the $50 (that staple, along with twenties, of ATMs) lasts for 6.1 years.