is the number (i.e., 1, 2, 3, etc.) in a cell?
If that number where in a cell (say cell A4), the formula would be =80+(A4-1)*20
for 1 in A4: 80+(1-1)*20 = 80
for 2 in A4: 80+(2-1)*20 = 100
If no cell specifies the number then you could use a different method. If this column (say column A) of your table starts at row 4, so A4 is the first cell, enter 80 into cell A4.
In cell A5 enter =A4+20
copy and paste the formula in cell A5 down the entire column and every row will increment by 20 from the previous row
You really know this well. Thank you for getting me started with the right answer and approach.