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
