Numbers 3:4

⁴ And Nadab and Abihu died before Jehovah when they offered strange fire before Jehovah in the wilderness of Sinai, and they had no sons; and Eleazar and Ithamar exercised the priesthood in the presence of Aaron their father.

Two and a half

In Exodus 25:10, this signifies much and full; when predicated of the Divine it signifies all. Its meaning is similar to that of five and ten.