Another way to look at this problem.

As I pointed out in the first reply, the grass will double in 3 days.

If it takes 2 cows to finish grazing in 6 days, then 1 cow would finish grazing 1/2 of the land in 6 day. What happens to the other 1/2 land? The grass on that 1/2 land has doubled twice/b in 6 days since every 3 days the grass doubles. That means at the end of the sixth day, the cow actually has twice the amount of grazing to do than on the very first day!

I hope this will give you a better picture of the problem the cow is facing. Know what you sign up for FIRST!!!

One muggle