A horse stable keeps 12 horses. Each horse eats 2 pounds of oats per day over a 2-day weekend. The same horses will be staying for 3 additional days. The stable manager estimates that the stable should have at least 7 pounds of oats for each horse for the 5 days.

What mistake did the manager make?

The manager did not make a mistake. The estimate is correct.
The manager calculated the amount of oats for 7 days instead of 5.
The manager calculated the rate per day for 1 horse instead of 12.
The manager estimated the remaining days at 1 pound per day instead of 2 pounds per day.
The manager determined that the horses received 1 pound per day for the first two days instead of 2 pounds per day for the first two days.