Find the side of the rectangle if its perimeter is 40 cm and the length is 12 cm. Find out the area of this rectangle.

Let’s denote the unknown width of the rectangle through x. According to the condition of the problem, the perimeter of the rectangle is 40 cm, and the length of the rectangle is 12 cm, therefore, the following relation is true:
2 * (x + 12) = 40.
We solve this equation:
x + 12 = 40/2
x + 12 = 20
x = 20 – 12
x = 8
Knowing the length and width of the rectangle, we can calculate the area S of this rectangle:
S = 12 * 8 = 96 sq. Cm
Answer: the area of this rectangle is 96 sq. Cm.