Let me try explaining this

The ROW function returns the row number of a single cell reference

ROW(reference)

The MOD function returns the remainder after number is divided by divisor

MOD(number, divisor)

Nested the ROW function in the MOD function which is to supply the number

argument, divided by 2 which is to sum every second cell will produced the

result

of 0

For example, to sum every second cell in the range A1:A20, the formula

could be this

=SUM(IF(MOD(ROW($A$1:$A$20),2=0,$A$1:$A$20,0))

Take note that this is an array formula which you must press Ctrl+Shift+Enter

at the same time to produce the desire result. Excel will add the curly

brackets to the formula

A better option is to use the DSUM function

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Hope this help

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Thank You

cheers, francis