Try this

=A1+SUM(IF(MOD(ROW($A$1:$A$100),10)=0,$A$1:$A$100, 0))

it's an array so commit with CTRL+Shift+Enter not just enter. If you do it
correctly Excel will put curly brackets {} around it.

Mike

"art" wrote:

Hello all:

Is it possible to sum the cells by starting from A1 and add A10 then A20,
and so on every tenth cell. How do I write the formula for it?