'Fifteen' in the Bible

Amaziah, the son of Joash, king of Judah, went on living for fifteen years after the death of Jehoash, son of Jehoahaz, king of Israel.

I will give you fifteen more years of life; and I will keep you and this town safe from the hands of the king of Assyria; I will keep this town safe, for my honour, and for the honour of my servant David.