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

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:   



We could rewrite that to say:



and then



So 

W = 12.5 - H

20000 as first it increases by 2500 then by 5000 so next it will increase by 10000