2339

This number is a prime.

There are 2339 ways to arrange the set of twelve pentominoes into a 6-by-10 rectangle, excluding trivial variations obtained by rotation and reflection of the whole rectangle, but including rotation and reflection of a subset of pentominoes. This case was first solved in 1960 by C. B. Haselgrove and Jenifer Haselgrove (now Jenifer Leech).