The piece of plywood is a square, 10 inches on each side.
Its area is (10 x 10) = 100 square inches.
You can cut a circle out of it with any size you want, as long as
the diameter of the circle is not more than 10 inches. You didn't
tell us what size circle, so I'll assume that you cut out the biggest
possible circle.
Diameter = 10 inches
Radius = 5 inches
Area of any circle is (pi) x (radius)²
Area = (pi) x (5)²
Area = (pi) x (25) = 78.5 square inches .
The rest of the plywood is scrap.
You discard (100 - 78.5) = 21.5 square inches.
(By the way ... you cut out a disk, not a circle.)