Trying to do this in Tasker need some help (Toll Booth) Variables

I already have Tasker set up that when I go through the toll booth that it announces that I have gone through the toll booth via autovoice.

What I would like to set a variable so that when I go through the Toll booth that it says I'm through the booth and subtract the amount of the toll from the beginning total and store that it a variable.
Example:
Go through toll booth

Tasker says: Your Toll has been paid and you have $XXX amount remaining in your account

I understand variables a little but not enough to do this

 

Just use a variable set action and check the box to "do maths" putting %var - tollcost