To sum every 2nd number in a vertical range...

=SUMPRODUCT(--(MOD(ROW(A3:A200)-CELL("Row",A3)+0,2)=0),A3:A200)

To sum every 2nd number in a horizontal range...

=SUMPRODUCT(--(MOD(COLUMN(A3:Z3)-CELL("Col",A3)+0,2)=0),A3:Z3)

Task Lead Nicole wrote:
Can I set up a formula in an xls to add up EVERY OTHER cell in a row? And how
long can a string in a formula be? (are there limitations?)

I'm getting an error when I manually type in the list of cells to add up.