Hi ^^
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A recipe calls for 3/4 cup of sugar and 2/3 cup of butter, so the sugar to butter ratio is (3/4)/(2/3) = (3/4)(3/2) = 9/8. This ratio implies that 1 cup of butter requires 9/8 cups of sugar.
Since you use 1½ = 3/2 cups of butter, you will need
(3/2)(9/8) = 27/16 cups of sugar.
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anyways hope that helped, bye! <3