The width of the rectangle is 8cm and its perimeter is 40cm. How long is the rectangle?

The perimeter is the sum of all the sides of the rectangle, that is, 2 widths and 2 lengths. Half of the perimeter is respectively equal to the sum of the two sides. If the width of the rectangle is 8 cm, then the length of the second side will be equal to the difference between half of the perimeter and the second side:

(40/2) – 8 = 20 – 8 = 12 cm.

Answer: the width of the rectangle = 12 cm.

You can decide otherwise.

Let the unknown side of the rectangle = X cm.

X + X + 8 + 8 = 40 cm;

2 * X + 16 = 40;

2 * X = 40 – 16 = 24;

X = 24/2;

X = 12 cm.