Answer
30 cm
Work Step by Step
$\frac{3}{2}$(perimeter of the printed area)=(perimeter of entire poster)
$\frac{3}{2}$(2$\times$140+2$\times$100)=2$\times$(140+2x)+2$\times$(100+2x)
$\frac{3}{2}$(280+200)=(280+4x+200+4x)
3$\times$240=480+4x
720=480+8x
240=8x
x=30
