Who knows thirty?
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30 are the years of the Levite draft (Num. 4:3).
30 letters (three times למשפחותיכם) is the width of each line in a Torah scroll. (Shulchan Aruch, Yoreh De'ah 272:2)
30 lines are used to write the Song of the Sea (Ex. 15:1-19) in a Torah scroll. (ibid. 275:4)
30 traits with which Malchut is attained (Avot 6:6)
A person who vows to be a Nazir without specifying for how long, has to remain a Nazir for 30 days. (Nazir 5a)
As we see in מדרש תנחומא חיי שרה סימן ו, thirty out of the seventy nations were scattered during the דור הפלגה. To make up for this, thirty nations came from Avrohom: the twelve sons of Yishmo'eil, the six sons of Keturoh, the two sons of Yokshon (the second son of Keturoh), the three sons of Dedon (the second son of Yokshon), the five sons of Midyon (the fourth son of Keturoh), and the two sons of Yitzchok (Yisro'eil and Edom, obviously!), which also explains a different question.