The length of a rectangle is twice it's width. If it's perimeter is 54cm, find its length. Explain!
2 L + 2 w = 54
so
w = 27 - L
also w = L/2
so
L/2 = 27 - L
3 L/2 = 27
L = 18
so
w = 27 - L
also w = L/2
so
L/2 = 27 - L
3 L/2 = 27
L = 18