Calculate the Item Quantity based on quantity of other Items in the Sales Order

Hi,

I have a requirement which needs the Item Quantity to be calculated based on other items of the sales order line.

Eg:

Sales Order has Item A [Qty 10], Item B [Qty 10]

When Item C is entered in the sales order the quantity should automatically get calculated and should be equal to the sum of Item A and Item B [20]. Also if Item A Qty is changed to 5, the Item C quantity should be changed to 15 as well. What is the best way to achieve this ?

Thanks