John Gill Commentary Genesis 18:29

"And he spake unto him yet again, and said, Peradventure there shall be forty found there. And he said, I will not do it for the forty`s sake." — Genesis 18:29 (ASV)

And he spoke [to him] yet again, and said

Being encouraged by such a gracious answer: Perhaps there shall be forty found there; forty righteous persons; and, from preceding passages, it must be supplied, "will you destroy and not spare the place for their sakes?"

and he said, I will not do [it] for forty's sake; but spare them for their sake.