one of your groups would have had to be between 50 and 100 for this to work, otherwise they can be swapped between either group, which makes it a non unique grid (if something can swap into another grid AND ALSO BE REPLACED it is no longer a red herring, which are what you want, but it is a non unique solution, which knackers the grid). I really hope this is useful to you and not offensive, you had some nice ideas, I'll be interested to see how you do in the future- keep trying, you had some really interesting and new things here :) Lorna x

BTW its a mistake I've made a lot of times too! I think most of us have at some point.