Numbers 3:13

¹³ for every firstborn is mine. On the day that I smote all the firstborn in the land of Egypt, I hallowed unto me all the firstborn in Israel, both of man and beast; mine shall they be: I am Jehovah.

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.