One hundred square feet is ten feet by ten feet on a side. Now, change each side to ten yards by ten yards. That means each side is thirty feet long. Thirty feet times thirty feet is nine hundred square feet.

3x3. They counted 1x1. 1/9. 8/9 is .8888

Easy math on divde by 9 1/9. .1111111 2/9 .222222 3/9 .333333 All the way to 9/9 where .9999 equals 1

The area is squared, so 1^2-(1/3)2 which is .888 or about 88.8%