yogi_Set Up Formula For Computing Sum of Last Specified Number Of Columns Of A Table

                                          Google Spreadsheet   Post  #1097
Yogi Anand, D.Eng, P.E.      ANAND Enterprises LLC -- Rochester Hills MI     www.energyefficientbuild.com.    Apr 7, 2013
user Jimmy Lucich :(http://productforums.google.com/forum/?zx=imdn03xd4ulc#!category-topic/docs/spreadsheets/7cBBo4DKGhA)
I'm trying to figure out how to add a column that only adds the last 10 (or whatever # I decide) columns on a sheet. 

So I would then have both my total of all columns and then this new "running average" of the last 10 columns only. Then, as I add data to empty columns, or as I add columns, only the 10 most recent (columns) of data are used and the 11th 'most recent' column would be dropped and not counted. As an example:

Suppose Columns A -- T. This "New column" would add only columns K--T. then, once column U is populated, this "New column" would add columns L--U, etc. 

Any formula, lookup that would do this would be most appreciated. Thanks for any help!

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following is a solution to the problem