Hi there!
I have attached two ways of solving this (in addition one of the solutions is splitted into alternative a and b and c)
In "Solution 1" you have to add a column with a formula to the right of your monthly data, see column E (you could of course also place it to the left of the data).
Perhaps you don't like that solution...
In "Solution 2a" you have to add two columns (with formulas) to the right of your quarterly data, see column F and G.
and perhaps you don't like that solution...
"Solution 2b" is actually doing the same as "Solution 2a". I just made everything more compact (but more difficult to "read") by integrating everything into one formula.
In all solutions, the formula in cell B37 is written in a general form, meaning it can be copied down and also can be copied to the right (to column D for example, as I did in the file).
I guess you are familiar with the technique referencing to an area like "$A$8:$A$34". There are several techniques how to avoid changing the "$34" to "$35" when adding one month. One simple technique is to have one blank month at the end, and always insert new month above this line (see "Solution 2c").
I hope this solves the issue as you wanted!
Kind regards
Bjorn
