Albert Barnes Commentary 1 Kings 11:42

"And the time that Solomon reigned in Jerusalem over all Israel was forty years." — 1 Kings 11:42 (ASV)

Josephus gave Solomon a reign of 80 years, either because he wished to increase the glory of his country’s greatest king or because he had a false reading in his copy of the Septuagint Version. It is, no doubt, remarkable that the three successive kings, Saul, David, and Solomon, should have each reigned forty years (Acts 13:21; 2 Samuel 5:4–5), but such numerical coincidences occur from time to time in exact history.