Sforno on Genesis 18:28/ספורנו על בראשית י״ח:כ״ח

1התשחית בחמשה את כל העיר, let me know if it is a definite rule of Your system of justice that unless there is a minimum of ten righteous people in a town You will destroy that town? If You were to do this, the result would be that the fifth city will be wiped out on account of five missing people! [based on the first Mishnah in Sanhedrin that only an עדה, quorum of ten male adults can “save” the accused. Ed.]

2ויאמר..לא אשחית, I will not destroy (the fifth).