Add Cells, but ignore those when text appears

Some data from tables have the word "INVALID" in them because this option is not available.
This word comes up in my formula and gives me #VALUE.
How do I get it to ignore the text "INVALID" and can I be warned if I enter a number in a cell that is linked to this "INVALID" text.

I have attached a Test file to try to explain what I want to achieve.
Hope it all makes sense to you.

Thanks All

Quagga, consider switching to SUMPRODUCT

C17:
=SUMPRODUCT(E4:G4,I4:K4)
copied down

obviously you should consider handling NAs also - to that end I would suggest using IFERROR in the I:K matrix itself (see XL Help for more info)

Originally Posted by Quagga

Is it possible to get an "Error Alert" when I want to type in a value in Cells E4:G13, but the relevant value in cells I4:K13 comes up as "INVALID"?

To kill as many birds with one stone as possible perhaps you could do the following...

Highlight E4:G13 having used E4 as starting point.. with the range selected apply Data Validation (via Data Tab on Ribbon):

Allow: Custom
Source: =ISNUMBER(E4*I4)