Room area parametrized

Hi, I'm trying to do a definition of a house which rooms are connected, for example to assign a slider that allows me to change the size of a rectangle (living room) and it affect the size of the other rectangles (rooms of the house)
I see something similar but I don't know which formulas in the evaluate he used.

Please any tip of where to investigate or solution will be great. Thank you.

Here's the example: https://vimeo.com/20281163