Hey Guys!!

Well, i have this table which has around 40 fields. The thing is that one of those fields has to be the sum of 20 others. I have been trying with no luck to make that particular field to be a calculated field of the sum of the other 20.. Can someone please help me out here? I really dont know why it isnt working. I've been using everything i've found but no luck.

It'd be much appreciated,