Summing

[fud18]

Ok I know some of you have gone over this, but here is the problem...I am
just trying to teach myself infopath so I don;t quite understand the formula
that you put out so that I can create my own total. I have set up an
expression box that figures the following:

(labor hours * hourly rate) + (parts qty * cost) = line total
Now I want to be able to have multiple lines like this and then create a
sum of the line totals for one big total.

If anyone can explain this in layman's term it would be much appreciated.

Thanks!