15.
“And it came to pass in the fourth year of King Hezekiah, which was the seventh year of Hoshea son of Elah king of Israel, that Shalmaneser king of Assyria came up against Samaria and besieged it” (2 Kings xviii, 9).
From the death of Ahaziah to the commencement of the siege of Samaria the reigns of Israel’s kings were: Jehoram 12 years, Jehu 28 years, Jehoahaz 17 years, Joash 16 years, Jeroboam 41 years, Zachariah and Shallum 1 year, Menahem 10 years, Pekahiah 2 years, Pekah 20 years, Hoshea 7 years. 12 years + 28 years + 17 years + 16 years + 41 years + 1 year + 10 years + 2 years + 20 years + 7 years = 154 years.
From the death of Jehoshaphat to the siege of Samaria the reigns of Judah’s kings were: Joram 8 years, Ahaziah 1 year, Athaliah 6 years, Joash 40 years, Amaziah 29 years, Azariah 52 years, Jotham 16 years, Ahaz 16 years, Hezekiah 4 years. 8 years + 1 year + 6 years + 40 years + 29 years + 52 years + 16 years + 16 years + 4 years = 172 years.
If from the death of Ahaziah to the siege of Samaria was one hundred and fifty-four years, and from the death of Jehoshaphat to the siege of Samaria was one hundred and seventy-two years, Jehoshaphat therefore died eighteen years before Ahaziah died.