Question

What is the width of an rectangle width of an rectangle with an perimeter of 25 cm

Answer 1

Impossible to give an absolute number but you could make an expression.   
   
If you take a rectangle, and count the height and width twice, you'll add up with the perimeter.     
    
So you could say:   

$P = 2W + 2H$

We could rewrite that to say:

$25 = 2W +2H$

and then

$2W = 25 - 2H$

So 

W = 12.5 - H