A dog on a 20-foot long leash is tied to the middle of a fence that is 100 feet long. The dog ruined

A dog on a 20-foot long leash is tied to the middle of a fence that is 100 feet long. The dog ruined the grass wherever it could reach. What is the area of the grass that the dog ruined.

The leash forms a circle where the dog can get to.
A = pi(r)^2
A = 3.1415(20)^2
A = 3.1415 * 400
A = 1256 square feet

The fence blocks off half the circle where the dog can move to, so we have a half-circle area:
A = 1256/2
A = 628 square feet