#### Answer

$b=12.7$ cm

#### Work Step by Step

The perimeter of a rectangle can be calculated as:
$P=2a+2b$ where a and b are the width and the length of the rectangle.
Here:
$b=2a-2.5$, $P=40.6$ cm
By substituting we get:
$40.6=2a+2(2a-2.5)=2a+4a-5=6a-5$
$40.6=6a-5$
$6a=45.6$
$a=7.6$ cm
$b=2a-2.5=2(7.6)-2.5=12.7$ cm