A number of people believe that theory.
The reality is that nobody knows, but there is a good chance that GMAC makes it harder to guess questions right, and has a rule (which in my mind makes sense - you want the abilities to decide the score, not a chance). I would probably see it closer to 10 than 20.
Not sure if this is the answer you were looking for, but I would say there i a good chance this is so. Plan to answer every question and spend some time on it. You will get the best/highest score if you don't have to guess any questions - that's the only way to guarantee it.
P.S. If you try to take GMATPrep - it does not have this behavior built into it, but it is missing other features as well, so don't base your GMAT understanding on the gmatprep software.
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