Find the shaded area.
Outer square area = 300 × 300
= 90,000mm^{2}
The four quadrants that have been cut out will form a circle with radius 100mm.
Area of circle = π × 100^{2}
= 31,415.93mm^{2}
Shaded area = 90,000 – 31,415.93
= 58,584.07mm^{2}