Calculation Help Needed

Hello I'm new here and this is my first post. I'm in the process of working on a project for school and i'm in the need of some help.

I have create a page in Photoshop and imported it into acrobat. The page is set up in the following way:

QTY Total

Package A

Package B

Package C

Sub Total

Tax

Grand Total

I'm need help in the calculation of the Tax Amount. I'm using the tax amount of 7.75%. I'm thinking to get the amount of take it will be the:

Sub Total * Tax (7.75)

The Fiels name are:

Sub Total = subtotal

Tax = tax

I have I explain this well enough to get some help.

Thanks