I keep meaning to share with you all one of my favourite puzzles, that being the Menseki, aka the Area Maze.
The only rule you have to know is that the area of a rectangle is length x width.
That’s it.
Here’s a great Guardian article here (with some examples to solve), and if you like them, there’s official apps for smartphones and the Switch to play.
Or you can get a book :)
EDIT: I forgot to say, you don’t need to use fractions nor decimals either. Literally just knowing the above rule.