Not counting rotations and reflections,
for the 3x20, there are but two solutions, the other
one derived from the one shown by rotating the section
outlined in white by 180 degrees.
"I"- or "+"-
based breakdowns, click on the P-pentominoes:
(some placements have unique solutions !)
the 2339 solutions of the 6x10
the 1010 solutions of the 5x12
the 368 solutions of the 4x15
the 6x10's in congruent halves
to refresh the sample solutions
of all four standard size puzzles.