The perimeter of the rectangle is 40 cm. One side is 5 cm. What is its perimeter.

1. To find the area of a rectangle, you need to know its length and width. Since the length is known, we find the width by the formula a = (P – 2b) / 2, where P is the perimeter of a rectangle with length b, width a:
a = (40 – 2 * 12) / 2 = 16/2 = 8 dm.
2. Find the product of the length and width of the rectangle, get its area:
Area = 8 dm * 12 dm = 96 dm2.
Answer: the area of the rectangle is 96 dm2.