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GMAT Algebra http://www.800score.com/forum/viewtopic.php?f=3&t=262 
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Author:  Gennadiy [ Sun Aug 18, 2013 7:06 am ] 
Post subject:  Re: GMAT Algebra 
questioner wrote: The correct answer should be 29 instead of 28: 15  (13) + 1, because 15 and 13 are both in the range inclusive. 15 and 13 are the maximum and minimum elements indeed, so to calculate the range (the difference between the largest and the smallest numbers in a set) we do not need to add 1, it is just 15 – (13) = 28 .Here is an example of applying the formula to a noninteger values: {3.5, 2, 0.5, 4.3} The range is 4.3 – (3.5) = 7.8 The minimum element is not necessarily negative, {2.5, 7, 15.4, 16.9} The range is 16.9 – 2.5 = 14.4 As also the maximum element is not necessarily positive, {6, 3.1, 2.5} The range is 2.5 – (6) =3.5 However, remember, that range is always positive or zero (in case when all elements are the same). 
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