updating stock levels using a formula

Hi,

was wondering what i need to do in excel to get this working:

In the same workbook as my other post i have three worksheets
1. is a transaction table
2. is a stock table
3. is a transaction type table

basically what i want to do is make the transaction table automatically adjust the number of items i have in stock for a given item:
An example of what i mean:
so i have 3 columns in the part of the transaction table i want to update, the Quantity, Transaction Type and the Total Stock Level

say i have item 1 and i have 3 in stock, what i want it to do is when i enter in the transaction type column of the transaction table - Into Stock, i want it to add what ever number i enter into the Quantity cell next to it onto the total stock level cell, and decrease the number if i enter - Sold.

The Total stock level is being held in the Stock table with all the other data for the items.

any help would be much appreciated as i have very weak knowledge of how the formula bits work in excel

cheers