We appointed for Moses thirty nights and completed them with ten, so the appointed time of his Lord was completed at forty nights. Moses said to his brother Aaron: "Be my successor with my people and reform, and do not follow the path of the corrupters."
Translation endnotes
[7:142] One of the reasons that the number "forty" is expressed curiously as "thirty plus ten" is the role that numbers play in the numerical structure of the Quran.