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COOK: About 1 hour before you’re ready to cook, remove prime rib from packaging; place on a baking sheet (larger side down) and allow to come to room temperature. Adjust rack to middle position and preheat oven to 275 degrees. Tent prime rib loosely with foil. Roast on middle rack until cooked to desired doneness, 1 hour and 30 minutes to 2 hours. Transfer to a cutting board to rest, at least 15 minutes.