how long and at what temperature do i cook a rib eye roast (20 lbs)?

Answer 1

Bring it to room temperature first.
Preheat oven to 475 F.
Put the roast in and immediately reduce the heat to 350 F.
Cook 18 minutes per pound (5 hours for your roast) for medium rare.

Bert

Answer 2

I've never seen a rib eye roast that big. Are you sure it's not something else!?! Anyway, most cookbooks recommend cooking rib eye roast at 350F. Internal meat temperature when you remove the roast from the oven is 140 for rare; 160 for medium and 170 for well done. My book shows 1 1/4 hours to 2 1/4 hours for a 4 to 6 pound rib eye roast. I don't even know how you can have a pan big enough for 20 # of rib eye, much less how big of a cow it had to have come from. Good luck.