It was our choice to extract the most difficult questions from the m13-m14, m17-m24 sets. So the population from which we chose the most difficult questions was the pool of m13, m14, m17-m24. It doesn't mean there are no difficult questions in other sets
I had the very same question.
Just a follow-up question though.. If the sets are not arranged according to difficulty, how come the 'most difficult problems' (the one located on the right side of the sets, in order of topic) is from sets 14 above? Doesn't this imply that the more difficult sets are the latter ones?
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