Question

You have 12 balls all look identical (in shape, color etc.). all of them have same weight except one defective ball

Answer 1

I think you want to ask probability question so,

no of balls we have=12
no. of defective ball=1

so the probability for getting that defective ball=1/12

hope this will help you,further query in comment section

Answer 2

probanility of defective ball is 1/12