can anyone simplify this equation please?

See if this does the trick - confirm with ctrl+shift+enter:
=SUM(IF(MOD(COLUMN(C7:AV41),3)=0,IF(C7:AV41="PAID",IF(MOD(COLUMN(D7:AW41)-1,3)=0,D7:AV41))))

Had to edit the post - previous formula was incorrect.

Did you get your colours sorted?

Hi TonyUK72, yes I did thanks, it's working a treat! njimack, I copied the formula above and I get #VALUE! for some reason???

Did you confirm with ctrl+shift+enter?

Ahhh......not sure what you mean by confirm with ctrl+shift+enter?

After you have entered the formula instead of pressing enter or clicking the tick, you have to press and hold ctrl + shift + enter. This is an ARRAY formula and that how you have to enter them.

Instead of simply pressing enter when the formula is inserted.....press CTRL + SHIFT + ENTER

That has indeed worked!! It's amazing what you find out on this forum, I had no idea about that! Very much appreciated njimack, thanks and I owe you a beer!

Perhaps you could help me with my final problem on the same sheet? As TonyUK72 mentioned above (and with the help of you guys on the forum), I have cells coloured depending on the 'age' of the 'due date' cell, so at the top of the sheet I have a summary of the number of cells (invoices) that are more than 7 days away from payment, within 7 days of payment, 1-7 days over, 8-14 days over and 15+ days over.

The above array function you created, adds up the paid invoices, but I'd also like to add up the totals in the same way for the above breaks. I've just tried substituting the "PAID" bit in the array formula with ">"&TODAY()+7), but it hasn't worked.

The code I have for the breaks above is as follows...

COUNTIF(B7:AY41,">"&TODAY()+7) for all 'current' invoices
COUNTIF(B7:AY41,">="&TODAY())-K4 for all invoices within 7 days of payment
COUNTIF(B7:AY41,"<"&TODAY())-COUNTIF(B7:AY41,"<40000")-O4-Q4 for invoices that are 1-7 days overdue
COUNTIF(B7:AY41,"<="&TODAY()-8)-COUNTIF(B7:AY41,"<40000")-Q4 for invoices that are 8-14 days overdue and
COUNTIF(B7:AY41,"<="&TODAY()-15)-COUNTIF(B7:AY41,"<40000") for 15+ days

Again any help greatly appreciated

For robustness's sake, control+shift+enter, not just enter:

=SUM(IF(MOD(COLUMN(D7:AW41)-COLUMN(D7),3)=0,IF(C7:AV41="paid",D7:AW41)))