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You need to be careful when buying stuff and be able to add up not only by number of say tins of beans in a pack but take in to account weight and other data to work out if something is cheaper or just to go with the non offer product.
@zleap @markusl yup. Classic example here in my regular shop is cat food. 88*100g pack of Felix (£30.49, Pets at home) is cheaper (per unit weight) than 80*85g pack of Whiskas (£24.99 in "cheap" supermarkets). But doing that calculation on the fly is beyond most people, the boxes look about the same size, and the individual packet weight is not easy to see.