Why use a half of box of cake mix to make a whole cake? If you mean you just want a small cake, you can buy a cake mix called Jiffy that makes something like a 8" cake layer. To make a cake with half of a cake mix you would have to measure out to see how many cups of mix in the package, then use half of that, and then do the same with the other ingredients, using just half of the number of eggs, oil, etc. However, that won't make a 9x13 cake, it would only make an 8" cake.
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