KöMaL 2011 K293

Fred's cows are grazing on a field. Each cow eats the same daily amount of grass, independently of the actual number of cows on the field. One day, Fred took 6 cows to the field. It took the cows 3 days to eat all the grass on the field, so at the end of the third day Fred had to withdraw his cows from the field to let the grass grow back. When the original amount of grass was restored, Fred took 3 cows to the field. He was surprised to observe that this time it took 7 days for the cows to eat up all the grass. He was so puzzled that he asked Ben who was good at maths. Ben reminded him that he had forgotten about something while watching the cows graze and counting the days. What did Fred forget, and how long would it take a single cow to eat all the grass in the field?