If you know both the perimeter an the length of a rectangle how can you determine the width of the rectangle?

The perimeter is the total of all 4 sides of the rectangle, and a rectangle, by definition, has 2 sets of parallel sides.

Perimeter = 2L + 2W (where L = length of ONE side, and W = width of ONE side).

So, for example, if the perimeter is 40 cm, and the length is 15 cm, you figure out the width by subtraction and division.

P = 2L + 2W

40 = 2(15) + 2W

40 = 30 + 2W

40-30 =30 + 2W - 30

10 = 2W

10/2 = 2W/2

5 = W

So the width of the rectangle is 5 cm.