What % do you define as "many"? Those percentages of content already known sound very high to me in regards to the first 1/3rd of the Sequences. (I'm still working on the rest so can't comment there.) Also, they can use the Article Summaries to test out whether they've seen the concept before and then read the full article or not. I don't recommend just reading the summaries though. I think a person doing that would be doing a disservice to themselves because of the reasons supplied by Vaniver above.