Let the radius of the big circle be r and the radius of the small circle be r,

So the area of the big triangle is R? Half of, the area of the small triangle is r? Half of

Shadow area (r? -r？ )half = 10

Know R? -r？ =20

The area of a ring is π(R? -r？ )=3. 14×20=62.8 cm2