66 is "six and three twenties" 77 is "seven and half four twenties"
I've been told that Danish accountants use Swedish or Norwegian numbers to avoid mistakes.
https://blogs.transparent.com/danish/2011/09/14/counting-in-...
(This happens a lot in Danish. It's incredibly sloppy on pronunciation and people somehow mostly still understand each other.)
EDIT: I had to think about it for a bit, but the situation for 77 is actually equivalent. No speaker of Danish actually thinks about "halvfjerds" as a compound number -- it just "70". I suspect it's actually very similar for French speakers, but their hyphens do make it harder to just lump the thing into a single concept/number :).
Did I somehow come up with "octante"? Is it not used in Belgium, or anywhere else? It's true that it sounds a lot more Greek (my native language) than "huitante".
Many French people believe that it is used in Belgium and Switzerland though, for some reason, but as far as I know that's not true (I live in Belgium, although not in a French speaking region).
I make a disservice to Pascale, the first lady who taught me French, and who was, herself French, by saying it was a Belgian woman who taught me French. The Belgian lady, Catherine, taught me _most_ of my French but I first started speaking French with Pascale.
Which is to say, it's possible I learned "octante" from Pascale, although she was far from old enough to have learned it in the early 20th century.
So I'm guessing "octante" is still used in some French-speaking places, although not that widely.
And I'll probably sound rather quaint if I use it myself :)
[For the record, I'm Greek but I learned French very early on because my dad had cousins in the Greek diplomatic corps and he wanted me to get a job there and he thought French is the language of diplomacy. I was not interested in that, of course. But at least, thanks to my dad's silly ambitions, I grew up practically billingual.]
Pronunciation is the same, unless there is a liaison: elle a quatre-vingts ans = there will be a 'z' sound between vingts and ans.
FYI, I'm a native speaker and I had to look the spelling part up to be sure. There are other rules with other exceptions for cent, mille, million, milliard.
70 is pronunced "60 10". 71 is "60 11". And so on until 80, which is pronounced, obviously, "4 20". 81 is "4 20 1". It goes up to 90, which is "4 20 10". 91 is "4 20 11". The craziness continues until 100, which is, thankfully, a different word ("cent", instead of "4 20 20", or, dear lord, "60 40"). But in 170 it all comes back.
I think the French-speaking parts of Belgium, Switzerland and Canada did the right thing when they replaced all that complexity by dedicated words for 70, 80 and 90.