I am keeping an inventory in Excel. Let's say I start with 100 units of widgets, I take 20 widgets from the shelf, but return 10 wdgets later because I didn't use them. I need a formula that shows the 100, then I want to show that I took 20, and then have Excel show that I returned 10. Can anyone help with this?

1. New excel sheet
2. Sheet 1: 1st row: Balance | =SUM(Transactions!A:A)
3. Sheet 2:
100
-20
10
Call sheet 1 Balance and sheet 2 Transactions.