Why in French numerals number "vingt" is being used to form 80 & 90 instead of more convential number formation related with nymber 8 and 9 respectively like in English?

It's somehow reminds me old English monetary system which was too difficult to understand, any connection with this?

marked as duplicate by Alexis Pigeon, Toto, Stéphane Gimenez Nov 8 '13 at 9:24

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