Calculating Field based on drop down selection

Good morning.  I got some great help earlier this week on a script to summarize costs and it works perfectly!

So, I thought I would take the chance to ask another JS coding question.  I have a drop down box (AttachmentType) with two options

* A (Out-of-State Travel) and

* B (In-State Travel, over $500).  

I have a field named SubTotal.

The field in question is my field labeled Plus5InState.

I need Plus5InState to calculate as follows:

If AttachmentType selection is "A (Out-of-State Travel)", then Plus5InState = $0

If AttachmentType selection is "B (In-State Travel, over $500)", then Plus5InState = SubTotal * 0.05.

Thanks in advance!

Ruth