I have an old lotus 123 spreadsheet (a budget) that used the @nsum function.
It allowed me to sum every third or forth column. The text below shows the
syntax. My question... does excel have a function like this?

Adds every nth value in list, starting at offset.
Syntax
@NSUM returns the sum of (offset), (offset + n), (offset + 2n), (offset +
3n), ... in list. @NSUM(0;1;list) returns the same result as @SUM(list).
Example
@NSUM(1;3;B5..B15) returns the sum of the values in B6, B9, B12 and B15.

Thanks in advance,

mh