Question: how long to cook 3.5 lb roast beef in oven?
Answer: Cook for 20 minutes a pound. A 3.5 lb roast takes about 1 hour and 15 minutes for a rare roast, but watch the thermometer reading. After you cook it for 20 minutes a pound, take it out of the oven, let it sit for 1 minute and then check the thermometer reading.