You can use an equation to solve this. Since the length is 4 more than the width, you can use L = W + 4. (L = Length, W = Width). I personally like to draw a diagram to figure out the total sides. We can add up all the sides, using our equations and adding common factors. We know the perimeter is 80 cm, so we can equal it to this. Solve for W. And finally subsitute for L.
