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We assume that prices of sandwiches aren’t increasing but only their costs because the factors of production used by the sandwich shop are also factors of production that can be used in other lines of production. For example, the sandwich shop may be using the same machine as the burrito shop to make their products. In that case if the new money is spent on burritos, the burrito shop can use that extra money to bid for more machines (the same machine used by the sandwich shop). When the sandwich shop goes to repurchase a machine, it will witness a rise in the price of the machine due to the extra money that got into the hands of the burrito shop.