These questions are all terribly worded. My best advice is just to skip them. The first question misuses the word 'range' (the range is the difference between the largest and smallest numbers in a set; they mean to ask for the interval of scores which are within one standard deviation of the mean); the second and third questions misuse the word 'series' (a series in mathematics is a sum, and we take the standard deviation of sets, not of sums), and regardless those questions make no mathematical sense. The GMAT will never test standard deviation in the way these questions are attempting to test it. I discussed what those questions are attempting to ask in a different post, and explained what takeaways are actually useful on the GMAT (scroll way down):
pls-explain-them-frnds-110985.htmlThe fourth question is ambiguous. First, sets are not in any order, so it makes no sense to talk about the 'first element' in a set. We also don't say 'twice greater' (technically that means 'three times as much', but you would never see that phrase on the GMAT); we just say 'twice'. Finally, in Statement 1, when they say 'consistent', it's not clear what that means. I think they mean to say that each set is equally spaced, in which case they should use the word 'equal', not 'consistent'.
I'm not sure where these questions are from, but if the original source contains questions like these, it will be more confusing than helpful to use it for your preparation.
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