If a figure can be tiled by N 2x3 rectangles it is trivial to place the same number of U pentominoes in that figure.
Find the smallest N such that at least N+1 U pentominos can be placed in a figure tiled by N 2x3 rectangles.
(In reply to posible solution 2
This is a slight modification of your solution. I merely rearranged the right half make the whole thing symmetrical and then used my notation to make it easier to see and compare to mine.
Posted by Jer
on 2016-03-09 18:05:13