OG10 was split into three books: OG11, the quant supplement and the verbal supplement. So some of the harder questions that are in OG10 are in OG11 now and some are in the quant supplement - you have to buy both books to get all of the hard questions.
The overlap between OG10 and the three newer books is about 75%. If you already have OG11, you might as well just buy the two supplements rather than OG10.
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Stacey Koprince
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Manhattan GMAT
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