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Why would it be best, considering the enormous cost of replacing 25 cent coins with 20 cent coins? What advantage does 20 cent coins have over 25 cent coins?

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Because it lets you round prices to 10 cents (drop a digit), rather than 5.

Technically you'd have to round to 20 cents, since you can't get 10, 30, 50 etc. cents from 20s.

We already have 10 cent coins.

The common pattern is 1, 2, 5, 10, 20, 50, ...

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