The area of the metal sheet is 20cm x 30cm = 600cm squared
The biggest circular sheet we can cut from the metal sheet, is a circular sheet having a radius of 10 cm
this circle has an area of pi * R^2 = 3.14159 * 100 cm^2 = 314.159 cm^2 so the remaining area of the sheet
is 600cm^2 - 314.159cm^2 = 285.841cm^2