Linking Worksheets - Changing Values on One Sheet based on a change on another

I have three sheets - One is an inventory summary sheet, one is an sales sheet and one is an orders received sheet. Each sheet has one column of item numbers and one column of which lists the number of items. For instance, A8 is item # 1100 and the number of items is in B8 - 2. The list continues like this. Each time an order is received a new column of number of items is added. For instance on May 2 and order was received and 3 of item # 1100 was received. In C8 the number of items is listed at 3.

I would like that each time a new order is received and the numbers of items are added in a new column on the Orders Received sheet, that it changes the number of the same item (in the corresponding cell) on the inventory worksheet.

Likewise, when sales of items are made, and a new column for the date of those sales is added to the Sales worksheet, I would like that to change the value in the corresponding cell on the inventory worksheet.

I'm not sure if this is the easiest way to do it, or if I can create some kind of dynamic inventory report for each time a change is made. Any help would be greatly appreciated.