How to make cells add and subtract a quantity from another cell when I change the value.

I have a TOTAL cell with 100 Apples and I want to distribute them to Jack and Jill.  If I “give” Jack 20 Apples and Jill 30 Apples, I want it to subtract from my TOTAL cell.  Likewise if I change the quantity of Jacks Apples to 15, I want it to distribute (add) the “extra” 5 Apples back to the TOTAL cell.

Thank you in advance.