Why Abraham Began His Plea for Sodom at Fifty Righteous

"Perhaps there are fifty righteous ones." And why did he open with fifty? Because there were five towns, so that there would be for each and every one ten righteous ones. And because in the generation of the Flood there were eight — Noah and his wife and his three sons and their three wives, behold, eight — and they were not able to save them; for this reason he said, "If I find in Sodom fifty righteous ones," and so forth.