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I agree with June and the Allen Browne reference. It also appears that you may not be showing all the things relevant to your inventory.
For example:
Do you have Customers/Clients, and Suppliers?
Do you do a physical stock taking/count periodically?
Most inventories deal with Transactions (transactionIncoming are +, transactionsOutgoing are -)
Quantity on Hand = Last Stocktaking Amounts + Transactions(Incoming -OutGoing)
Good luck.