A factory needs to deliver a batch of goods to a client. The transportation to the client’s location takes 15 days. Upon arrival, the client needs 3 days to inspect and accept the goods. Due to some unforeseen delays, the factory will need an additional 5 days to prepare the shipment before dispatch. If the client must receive the goods exactly 10 days from now, how many days ago should the factory have started preparing the shipment? Let's think step by step and output the final answer within \boxed{}.
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