To find the area of the shaded region, we can find the area of the total region, then subtract the non shaded part.
Area of total region:
A = 20cm x 30cm = 600cm^2
Area of non-shaded region:
A = 5cm x 18cm = 90cm^2
Subtracting the non-shaded from the total region:
600cm^2 - 90cm^2 = 510cm&2.
Therefore, the answer is 4. 510cm^2