1. viginti — de Vaan
viginti 'twenty'. The higher decades have adopted -aginta from 'thirty' and 'forty'. The expected form of 'fifty' would have been *penkwe-dJont-h2 > *quinqueginta9 of 'sixty' *sexginta/*seginta. Octoginta is regular and nonaginta probably for *nunaginta < *hineun-dkrnt-. The number septuaginta is explained by Meiser from remodelling of *septmaginta < *septw-h}kmt-h2> in other words, he assumes a prior development to … — [de Vaan, s.v. viginti, p. 276]