Say your data is in column A and in column B you enter:

=ISEVEN(ROW()+1)*1 and copy down. Then =SUMPRODUCT(A1:A100,B1:B100) will
get your desired result for 100 items.
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Gary's Student


"Sarah needs to know" wrote:

I have a list of numbers all in one column and I want to be able to type in a
formula that will SUM the numbers in every second cell only, without having
to click on each cell individually. This is how I do it now,
=sum(a1+a3+a5+a7+a9+....etc). Is there a formula that will tell excel to do
this automattically? If so can you tell me how to do it?