The area of the rectangle ABCD is 21 cm2. The width of the rectangle is 30 mm. Calculate the perimeter of this rectangle.

Let’s translate into a single measurement system, for example, in centimeters: 30 mm = 3 cm.
As you know, the area of a rectangle is calculated by the formula: S = a * b, where a is the length of the rectangle, b is the width of the rectangle.
Substitute all known values into the formula and calculate the second side of the rectangle: 21 = a * 3.
a = 21/3 = 7 cm.
As you know, the perimeter of a rectangle is calculated by the formula: P = (a + b) * 2, where a is the length of the rectangle, b is the width of the rectangle.
We calculate the perimeter of the rectangle: P = (7 + 3) * 2 = 14 + 6 = 20 cm.
Answer: the perimeter of the rectangle is 20 cm.