Subtracting a value from another form

Subtracting a value from another form

Hi,
I have a form called performances which has 3 numerical fields:
  1. Number_of_Tickets_Available
  2. Number_of_Tickets_Allocated
  3. Tickets_Remaining
I have another form called Tickets_Allocated. This form pulls the values of the three numerical fields from the performances form. This form also has a numerical field called:
  1. Number_of_Tickets
The user will add a number into this numerical field. When I submit the form Allocate_Tickets, I want to add Number-of_Tickets to the Number_of_Tickets_Allocated field on the performance form. This then subtracts from the Number_of_Tickets_Available to give the Tickets_Remaining value.

As I am allocating tickets, not all tickets will go to the same customer, so I might have 100 tickets available and allocate 10 to customer A. Then I might allocate 15 to customer B and so on.

Is there a way to do this?