Avraham (Abraham) asked Hashem to save the cities of Sedom, Gomorrah, Admah, Zeboiim, and Zoar. Hashem agreed to save them if there were 50 righteous people between all of these cities. According to Rashi each city required the same number of righteous to save it [Bereishis (Genesis) 18: 24, 28]. How many righteous people would be necessary to protect each city?
50/5 = 10 righteous people
Hashem then agreed to save them if there were only 45 righteous people in the cities. How many righteous people would be necessary to protect each city?
45/5 = 9 righteous people