Why did Thai Language borrow Middle Chinese for numerals 2-4 (song, sam si), 7-10 (chet, pet, kao, sip), 20 (yi sip) & -1 (et, in higher numerals) whereas numerals 5 (ha < *lŋa / *sŋa) & 6 (hok < *krok ) seem to be Old Chinese loanwords?

Why did Thai Language borrow Middle Chinese for numerals 2-4 (song, sam, si), 7-10 (chet, pet, kao, sip), 20 (yi sip) & -1 (et, in higher numerals) whereas numerals 5 (ha < *lŋa / *sŋa) & 6 (hok < *krok ) seem to be Old Chinese loanwords?

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