Can somebody me with number 20? !

Choice B, 8 square cm



Explanation:

A1 = shaded area inside larger rectangle 
A2 = shaded area inside smaller rectangle
A3 = unshaded overlapping region

A4 = A1+A2 represents the area of the entire 6 by 8 rectangle
A4 = 8*6 
A4 = 48

Similarly,
A5 = A2+A3 is the area of the entire 4 by 5 rectangle
A5 = 4*5 
A5 = 20

So we know that
A4 = A1+A3 = 48
A5 = A2+A3 = 20

Or in short,
A1+A3 = 48
A2+A3 = 20

Subtract the equations
A1+A3 = 48
A2+A3 = 20

A1-A2 = 28

The total shaded region is 52, which means,
A1+A2 = 52

The new system of equations is
A1-A2 = 28
A1+A2 = 52
which combine to 2A1 = 80 and that solves to A1 = 40

If A1 = 40, then A2 = 12 after using A1+A2 = 52

Then we use A1+A3 = 48 to find that A3 = 8