Hashem Listens

Why does Hashem wait for Avraham Avinu to argue on behalf of Sedom to such an extent ?

Maybe there are 50 righteous ?

45 ?
40 ?
30 ?
20 ?
10 ?
Hashem could have said right away - Sorry ... ?
Now may be a good time for a gem of Torah study.
www.GemsofTorah.com