.
.
.
  The best way to approach this is to look for common characteristics among the puzzles and then see if you can match up specific letters to go with these characteristics.
.
.
The letter O must represent the circles.
The letter F must represent two.
The letter X must represents three.
The letter U must represents squares.
The letter R must represent figures
       that are seperated.
The letter L must represent figures
       that are linked.