Entered it where..in what event?
First, you have to understand that Anything + Null (an empty Textbox) = Null. To have Service Cost or Parts Cost entered in the Total Cost Textbox, you need to allow for one of them being empty...so, in VBA in an event (in the AfterUpdate event of the Service cost and Parts cost Controls, you'd use
[Total cost] = (Nz([Service cost],0)) + (Nz([Parts cost], 0))
which converts a Null value to Zero.
Or, in the Control Source Property (in Properties - Data) for [Total cost] you'd use
= (Nz([Service cost],0)) + (Nz([Parts cost], 0))
The latter suggestion is probably the one you should be using.
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007