Find the perimeter of rectangle ABCD with sides AB = 15 cm BC = 20 cm.

From the very beginning, we should remember the formula that is needed to determine the perimeter of a rectangle:
P = 2 * (a + b),
where P is the perimeter of the rectangle;
a – the length of the rectangle;
b is the width of the rectangle.
In accordance with the condition of the given task, in the rectangle ABCD, the length (a) is 20 cm, and the width (b) = 15 cm.Consequently, the perimeter of the rectangle with such parameters will be equal to:
P = 2 * (a + b) = 2 * (20 + 15) = 2 * 35 = 70 (cm).
Answer: the perimeter of the rectangle is 70 cm.