Well there isn't anything weird with the counting or mathematical usage, just the names for them seem not to use a typical decimal system. For example, in English and Norwegian, 75 and 82 are straightforward compounds of "7,5" and "8,2". In English, seventy-five, eighty-two; in Norwegian, syttifem, åttito.
But in Danish, 70 isn't syv-something. Instead it seems it's halvfjerds, which sounds like half a fjerd, except a fjerd is not a number. ;-) And even more confusingly, 50 sounds awfully like half-60.
From what I've read, it used to be "half four twenty" (halvfjerdsindstyve), where "half four" is an archaic way of saying 3½, so 3½ 20 = 70. Which later, I suppose, got contracted into halvfjerds. Admittedly you can just treat them like opaque symbols to memorize, so I doubt anyone thinks of 75 as "5 and 3½ x 20". But the Norwegian numbers seem more orderly...
That's sort-of similar (though even weirder than) French[1], where, where you sometimes count in twenties instead of tens. So instead of sixty-nine, seventy, seventy-one, it's (the equivalent of) sixty-nine, sixty-ten, sixty-eleven. And eighty is really four-twenties, so ninety-five is four-twenties-and-fifteen, etc.
Of course, English used to be like this too — thus "three score years and ten". And many languages still retain non-decimalised words for eleven and twelve (and sometimes powers of them, like a gross or mass.)
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[1] Well, French French. Swiss French and Belgian French are much more sensible.
Ah, yes that is weird once you start thinking about it. As a native speaker I've never given it a second thought, but I can certainly see how it would be confusing.
