The width of a rectangle is 12 units less than its length. If you add 30 units to both length and width, you double the perimeter. Find the length and with of the original rectangle.

length = width+12

(w+30)+(w+12+30) = 2(w+(w+12))
w=24

check:
24+36 = 60
(24+30) + (36+30) = 54+66 = 120