If we must do 13 minutes of roasting time for every pound of beef, and we have 11.5 pounds of roast beef, the total roasting time will be the multiplication:
Then it should stay 149.5 minutes in the oven. But it's not a practical result, then, let's convert in hours and minutes:
Then 149.5 minutes is equivalent to 2 hours, 29 minutes, and 30 seconds. We can do an approximation and say that the roasting time is 2 hours and 30 minutes.