Maximum Hole Problems
Here the problem is to find a figure in which the maximum number of unit holes can be placed. This problem was fist suggested for the pentominoes by Stephen Barr and pubicised by Martin Gardner in Scientific American in September 1972. Later Pieter Torbijn proposed finding rectangles with the maximum number of holes.
The links below lead to various sections of the site where solutions to this problem may be found.
Polyominoes Polyiamonds Pentominoes Hexiamonds Hexominoes Heptiamonds
Polycubes Polyhexes Pentacubes Pentahexes Solid hexominoes
Other Polyforms Tridominoes Strip-hexominoes