Heat oven to 350 degrees. Prepare and bake cake as directed on package for a 13-by-9-by-2-inch rectangular pan. Cool 15 minutes.
Stir together lemonade concentrate and powdered sugar. Poke warm cake with fork at 1/2-inch intervals, wiping fork occasionally to reduce sticking. Drizzle lemonade mixture evenly over cake. Refrigerate cake until cold. Spread cold cake with frosting. Cover and refrigerate.
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