Merging cells mid table in Numbers

It does not appear that you need the first two columns to be header columns. Make them regular columns then you can merge the columns as you wish. With the table selected, look at the sidebar (the thing with the settings on the right side of the window) and click on the Table tab. Change the number of header columns to zero. You may have to reformat the cell background of the two columns to turn them gray again. You can use Copy Style and Paste Style to get the style from a cell in the header row.

Neil Paisnel wrote:

explanation of the calculations below it.

Have you considered putting the explanation or explanations in a comment or comments (click a cell and from the menu choose Insert > Comment, or type shift-command-k)?

Comments are a good place to put information like this. Merging cells is not always a good idea because it can produce unexpected results in calculations.

SG

You can edit a post only within a few minutes of posting it. You can click the down-arrow at the bottom of the post and report it and a moderator can remove the image for you. I reported it for you but you can do it too, just to be sure.