The perimeter of a rectangle is 78 cm what is the length of this rectangle if it is known that its width is 12 cm.

To solve this problem, remember that the perimeter of a rectangle is the sum of the lengths of all its sides. Since in a rectangle the opposite sides are equal, then P = 2 * (a + b), where a is the length, b is the width. Knowing that the perimeter is 78 centimeters and the width is 12 centimeters, we calculate the length.

(a + 12) * 2 = 78;

a + 12 = 78/2;

a + 12 = 39;

a = 39 – 12;

a = 27;

Answer: 27 centimeters.