Answer: the widht is 5 cm and the lenght is 10 cm.
Explanation: The question says:
" If the length of a rectangle is twice its width and the perimeter of the
rectangle is 30 cm, what are the length and width of the rectangle?"
So we know that:
L = 2W
and
2L + 2W = 30cm
replacing the first equation in the second one:
4W + 2W = 30cm
6W = 30cm
W = 30cm/6 = 5cm
and L = 2W = 2*5cm = 10cm