2/3 is a simple concept. It refers to those questions in which 2 options leads to one direction whereas other 3 option leads to another direction. So in such cases you can choose either one of the direction and can half your options to choose the correct answer.

For example:-

Check question number 14 of OG 11. In that question primary elimination factor is to choose between "OR" or "AND", or in short you divide the options in 2/3 splits.

Hope this clear your doubt...

Regards,

Thanks codesnooker