The weather is even.

Assuming that each cow eats 1 serving of grass every day,

The reduction rate of grass is:

(20×5- 16×6)÷(6-5),

=4÷ 1,

=4 (copies);

Number of original grass on grassland:

20×5+5×4,

= 100+20,

= 120 (copies);

Then 1 1 a cow eats 1 1 a portion of grass every day, and 4 portions of grass are reduced every day, which can be regarded as (1 1+4) a cow eats grass every day, and the original 120 portions of grass can be eaten.

120÷( 1 1+4),

= 120÷ 15,

=8 (days);

Answer: You can feed 1 1 cow for 8 days.