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Sum last n columns

Excel formula: Sum last n columns
Generic formula 
=SUM(INDEX(data,0,COLUMNS(data)-(n-1)):INDEX(data,0,COLUMNS(data)))
Explanation 

To sum the last n columns in a table of data (i.e. last 3 columns, last 4 columns, etc.) you can use a formula based on the INDEX function. In the example shown, the formula in K5:

=SUM(INDEX(data,0,COLUMNS(data)-(K4-1)):
INDEX(data,0,COLUMNS(data)))

where "data" is the named range C5:H8

How this formula works

The key to understanding this formula is to realize that the INDEX function can be used to return a reference to entire rows and entire columns.

To generate a reference to the "last n columns" in a table, we build a reference in two parts, joined by the range operator. To get a reference to the left column, we use:

INDEX(data,0,COLUMNS(data)-(K4-1))

Since data contains 6 columns, and K4 contains 3, this simplifies to:

INDEX(data,0,4) // all of column 4

To get a reference to the right column in the range, we use:

INDEX(data,0,COLUMNS(data))

Which returns a reference to column 6 of the named range "data", since the COLUMN function returns 6:

INDEX(data,0,6) // all of column 6

Together, the two INDEX functions return a reference to columns 4 through 6 in the data (i.e. F5:H8), which resolve to an array of values inside the SUM function:

SUM({15,14,10;9,12,12;7,9,9;12,13,13})

The SUM function then calculates and returns the sum, 135.

Author 
Dave Bruns

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