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 by armando)

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.

      AABB
CCDD  AABB
CCDDEEAABB
CCDDEEFFGG
HHIIEEFFGG
HHII  FFGG
HHII

      JJJX
XKKK  JLJL
MKMKNNNLLL
MMMONONPPP
QQQOOORPRP
QSQS  RRRX
XSSS

Posted by Jer on 2016-03-09 18:05:13