One square yard is the same as 3 feet by 3 feet, or 3*3=<<1*3*3=9>>9 square feet.
Ten square yards is 9*10=<<10*9=90>>90 square feet.
Therefore, 100 rabbits can clear 100*90=<<100*90=9000>>9,000 square feet of grassland per day.
A 200 ft by 900 ft area is 200*900=<<200*900=180000>>180,000 square feet.
Thus, 180,000 sq ft can be cleared by 100 rabbits in 180,000/9000=<<180000/9000=20>>20 days
#### 20