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There is not only one way to form rectangles from the 12 pentominoes. Among the trillions of possibilities to arrange the twelve pentominoes in a 6x10 rectangle, there are in excess of 2000 'solutions' which fill the rectangle exactly. According to Arthur C. Clarke's 'Imperial Earth' there are 2,339 solutions.
Also asked, how many Pentominoes are there?
twelve pentominoes
Similarly, how many Polyominoes are there?
It has been shown, though, that there are 35 types of hexominoes (composed of six squares) and 108 types of heptominoes (seven squares), if the dubious heptomino with an interior “hole” is included. The term polyomino was introduced in 1953 as a jocular extension of the word domino.
In addition to pentominoes there are also so-called 'pentacubes'. These are the 29 truely three-dimensional shapes formed from 5 cubes that touch each other on at least one face.