How do you make 12 cupcakes out of 24 cupcake-cake mix?

I have Baker Josef's (Trader Joe's) golden yellow cake mix that can make 24 cupcakes. I would only like to make 12 cupcakes and was wondering if anyone had a solution to this problem (it calls for 1 cup of water, 3 eggs, 1 cup oil/1 cup melted er).