"Saith deg tri"
deg is the number ten, decimal numbers are simply another number and ten
eg thirty = tri deg (lit three ten)
forty = pedwar deg (lit four ten)
seventy three = saith deg tri (seven ten three)
The decimal system was devised in the second half of the twentieth century as Welsh medium schools started using Welsh to teach Maths.
The traditional Welsh number system is vigesimal (based on twenty) and is a bit cumbersome to do maths with.
eg. thirty = deg ar hugain (lit ten on twenty)
forty = deugain (two score in English)
seventy three = tri ar ddeg ar drigain = (three on ten on three score)
Yes, especially for the smaller numbers, also 'hugain' = twenty, 'pump ar hugain' = twenty five are still used for time.
The ordinal numbers are based on the traditional number system is still strong and is taught in schools and are especially important for dates.
e.g today is 'Dydd Gwener, Mai Y Seithfed ar Hugain (Friday, May the twenty seventh (lit seventh on twenty))