#### Answer

30 cm

#### Work Step by Step

We know that the perimeter is 2*length+2*width. The perimeter of the poster is:
Poster perimeter=$2*(140+2x)+2*(100+2x)$
The perimeter of the printed area is:
Printed area perimeter: $2*140+2*100$
We know that the poster perimeter is 1.5 times the printed area perimeter:
$2*(140+2x)+2*(100+2x)=1.5*(2*140+2*100)$
We solve for $x$:
$280+4x+200+4x=1.5*(280+200)$
$480+8x=1.5*480$
$8x=0.5*480=240$
$x=240/8=30$
So the blank strip is 30 cm wide.