Sorry for not being clear I am editing my question, I myself had not read it thoroughly, (was tired at the time)...
This is the exercise:
There are 20 items in total. Among them there are 8 failed items. We choose randomly 2 items. What is the proability that these 2 items are both failed.
Soltion (in the book):
Threre are C(20,2) ways we can choose 2 any items.There are C(18,2) ways we can take 2 failed items. And the probability is C(18,2)/C(20,2).
I am sure this is trivial exercise containing a typographical error, and we should have C(8,2) insted of C(18,2), right?
Thank you very much!
My aplogies again for not being clear!