Here is one option:

To sum every second column
=SUMPRODUCT((A1:AA1)*(MOD(COLUMN(A1:AA1),2)))

To sum every second row
=SUMPRODUCT((A1:AA1)*(MOD(ROW(A1:AA1),2)))

Change 2 to 3 to sum every third row.

Hope it helped
Ola Sandström


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