The official guide consists of actual retired GMAT questions though they are somewhat old.
1. The OG 12
is the most representative and closest source of GMAT questions to the real GMAT.
2. The questions are quite representative though there are always reports that people encountered questions they have not seen in the OG, which is to be expected
3. The way to get a great GMAT score is not by studying each question type in the OG 12
but by knowing the basics and concepts behind them. it is possible to get a great score even without using the OG 12
4. The questions in the OG 12
are arranged in an increasing order of difficulty, so depending on what your score is, you should be looking either closer or further to the end to get an idea of the level and types of questions, though again, you will never see the exact ones on your test and will always have something new.
See this one: gmat-club-guide-to-the-gmat-official-guide-12th-ed-85956.html
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