X = Ancho Original
X + 20 = Largo Orginal
Perimetro = X + (X + 20) + X + (X + 20) = X + X + X + X + 20 + 20 = 4X + 40
Perimetro Original = 4X + 40
Ancho se reduce en su tercera parte
X - X / 3 = 3X / 3 - X / 3 = 2X / 3
Ancho Nuevo = 2X / 3
Largo se reduce a la mitad
(X + 20) / 2 = X / 2 + 10
Nuevo Perimetro = (2X / 3) + (X / 2 + 10) + (2X / 3) + (X / 2 + 10) = (4X / 3) + X + 20 = 7X / 3 + 20
5 / 9(4X + 40) = (7X / 3) + 20
20X / 9 + 200 / 9 = 7X / 3 + 20
200 / 9 - 20 = 7X / 3 - 20X / 9
200 / 9 - 20 = 200 / 9 - 180 / 9 = 20 / 9
7X / 3 - 20X / 9 = 21X / 9 - 20X / 9 = X / 9
20 / 9 = X / 9
20x9 = 9X
X = 20 metros
Ancho = 20 metros
Largo = 20 + 20 = 40 metros
Perimetro Rectangulo Original = 4(20) + 40 = 80 + 40 = 120 metros.