Jamieson, Fausset and Brown on 2 Kings 15:30

30. Hoshea the son of Elah made a conspiracy ... and slew him--He did not, however, obtain possession of the kingdom till about nine or ten years after the perpetration of this crime [Hales].

in the twentieth year of Jotham--Jotham's reign lasted only sixteen years, but the meaning is that the reign of Hoshea began in the twentieth after the beginning of Jotham's reign. The sacred historian, having not yet introduced the name of Ahaz, reckoned the date by Jotham, whom he had already mentioned (see 2Ch 27:8).

2Ki 15:32-38. Jotham's Reign over Judah.