Answer:
4 cm
Explanation:
To calculate the area of a rectangle, we find the product of its width and length. This means that to determine the width or length of a rectangle, we can divide the area by the other dimension (length or width) accordingly. Therefore, the formulas are:

Note that in a rectangle, the longer side is referred to as "length" and the shorter side as "width".
Starting with the green rectangle, we can calculate its width by dividing its area (12 cm²) by its length (4 cm):

Now we have one side of the yellow rectangle, we can calculate the length of the other side by dividing its area (9 cm²) by 3 cm:

(The yellow rectangle is in fact a square, since its length and width are the same).
The length of the purple rectangle is the sum of the lengths of the yellow and green rectangles. Therefore:

To find the width of the purple rectangle, divide its area (14 cm²) by its length (7 cm):

The width of the red rectangle is the sum of the widths of the yellow and purple rectangles. Therefore:

To find the length of the red rectangle, divide its area (25 cm²) by its width (5 cm):

(The red rectangle is in fact a square, since its length and width are the same).
The length of the blue rectangle is the sum of the lengths of the red and purple rectangles. Therefore:

Finally, to find the width of the blue rectangle, divide its area (48 cm²) by its length (12 cm):

Therefore, the width of the blue rectangle is 4 cm.