Tetzaveh: The Chosen

The Talmud tells us that the choshen of the Kohen Gadol had the names of the 12 tribes along with the names of the three Avos. It also had the words Shivtei Yeshurun. It is taught that these 72 letters were divided evenly among the twelve stones on the choshen [The Baal HaTurim on Shemos 28:17].

Question: How many letters were on each of the stones? 

Answer: 72 / 12 = 6 letters on each.