Hi there.
I have joomla 2.5 and community answers 2.x and all I want is as simple as this.

Only registers users should be able to ask questions and answer questions, guest can view questions and answers, but cannot ask and answer.

I tried several combinations, nothing seems to be working,

I see several had permission questions, it would help if you can share basic permissions screen shot or expose your backend where in we can look at how you have set up.

Thanks for your help.