Apostol, Mnatsakanian: New Horizons in Geometry :
See also: https://en.wikipedia.org/wiki/Visual_calculus
There are a few problems where I wish they'd be clearer about what the number labels mean. Areas? Edge lengths? They look like they're labeling areas, but I assume that figuring out what the numbers mean isn't intended to be part of the puzzle.
#3 was particularly elegant, once I saw the trick. I got x=5 for it.