Didnt get much time to think on this, but what i can think about is - if you can send in batches like 2 at a time or 3 at a time and each has a different set of symbols so you can only save one or two. You also could make it such that if you save one, the rest automatically disappear. Now you can add a layer to this where there is some risk reward payoff like an easy symbol has low points and a difficult set of symbols has more points. Something on those lines. I actually just thought of a puzzle twist while writing this. :D so lets say there are 3 ghosts. First has a dash symbols second has a dash and circle and the third has 2 circles and a dash. Now if you draw a dash, just 1 will get saved and the rest 2 will disappear. But if you do a circle first and then a dash then you can save 2! And if you do 2 circles and then a dash, you can save 3. So the order matters.. hope i was able to communicate this well:D