Ms Access Problem

[Les Cooper]

Hi All

I hope that someone can help!!!!!!!
I have a fuel tank and obviously sell fuel and add fuel to the tank. the problem I have is

I have an initial start amount in the tank. I sell some fuel and that leaves me the start amount - the fuel sold. I wish to display how much fuel I have remaining on my form when I create a new record. I then wish to add fuel to my tank which will then display how much fuel I have remaining again.
Basically I want to display the amount of fuel left in my tank after transactions. I have tried many times, video's and forums but continually hit a brick wall when I attempt to do this. I'm conversant with Access but cant solve this one.

I will be using Access 2007-2010 and possibly access 2013.

Thanks for taking the time to read and possibly help.