I just finished doing preptest 33. I've done all of the tests in order so far, plus a number of the higher-numbered ones. I think that the low numbered tests are great for study. They are really good if you're starting out because they approach things more concretely. It makes them easier at a low level. At this point, with myself scoring pretty high, I am finding them harder because they are less consistent than the low numbered tests in terms of how and what they ask.
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I saw some on amazon.com going for $20-25 per 10 PTs.