Hi didieravoaka,

There are a variety of sources that you can go to to find explanations for these questions. If you try 'googling' any of them, you'll almost certainly come across links to various websites, videos, etc. that will walk you through how to solve each of these questions.

It's worth noting that the OG will typically offer a 'technical' explanation for any given question (and not necessarily the fastest or easiest approach), but most questions that you will see on Test Day can be solved in a variety of different ways. Thus, to maximize your performance/pacing, you shouldn't just focus on one approach to a given question - learning alternative methods, tactics, patterns, etc. is an important part of the study process.

GMAT assassins aren't born, they're made,
Rich