From the given diagram we get that:

Subtracting 4x from the above equation we get:

Adding 1 to the above equation we get:

Dividing the above equation by 6 we get:

Therefore, the length of the given rectangle is:
![10\cdot(1)/(2)-1=5-1=4\operatorname{cm}\text{.}]()
Now, since the area of a rectangle is the length times the width, we can set the following equation:
![width*4\operatorname{cm}=20cm^2\text{.}]()
Dividing the above result by 4cm we get:

Answer: The width of the given rectangle is 5cm.