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Since the changed recipe calls for 2 1/2 cups of flour and sugar combined, and the original recipe calls for 2 cups of sugar, the difference between the two is the amount of flour in the original recipe.
Hence, the amount of flour in the original recipe is 2 1/2 cups - 2 cups = 1/2 cup.
Therefore, the amount of flour in the original recipe is 1/2 cup.