设宽为X 则长为2X+4
(2X+4+X)x2=40
(3X+4)x2=40
6X+8=40
6X=40-8
X=32/6
X=16/3
2X+4=32/3+12/3=44/3
长为44/3厘米,宽为16/3厘米.