One way:

Manually:

A1: 101
A2: 121

Select A1 & A2, drag the fill handle (the lower right corner) down as
far as required.

By Formula:

A1: 101
A2: =A1 + 20*(ROW()-ROW(A1))

copy down.

In article ,
"boatbuilder" wrote:

i want to be able to add 20 to 101+20 as an example, therefore the result in
the cell would be 101+40, then copy it so it would go on 101+60, 101+80 etc.