Update: OK I'm re-reading everything and I'm still not sure that I understand it after all.. *sigh*..

For those that don't get it, like I didn't, realize that we're making one important assumption here: that both parties know the upper and lower bounds of the numbers!

Sure, if we know we're working from 0 .. 100, and the envelope we see is number 88, odds are, it's the higher of the two envelopes (assuming both are truly random; this can be defeated if the "dealer" keeps writing two consecutive numbers or otherwise knows the trick). Now I look at the situation and think, "Well yah, duh."

But what if you don't know the upper bounds? Modify the lines in the script like below, and the odds come out to exactly 50%: what we'd expect. The way *I* would play this game in real life, without knowing the limit, is to try to "figure out" the limit as I go, but statistically, given a one-time shot, we have to assume that the number we were given is, on average, mid-way through the distribution. This means that no matter what number we think of, we're still 50% likely of guessing right.

...
  $letter1 = $money[rand @money / 2];
  { $letter2  = $money[rand @money / 2]; redo if $letter1 == $letter2;
+ }
...
  { $guess = $money[rand (($letter2+1) * 2)]; redo if $guess == $lette
+r2; }
...
[download]

But if we know we're going to play 100 games, we can sorta "figure out" the upper limit as we go (assuming there is one, and it's kept constant during play), by assuming that our numbers will average out to be mid-way through the distribution. That would let us approach a higher probability of being correct further down the line. The script can be modified to play by those rules too, but I'll leave that as an exercise for the reader.

If you were to actually make these bold statements and try them in real life (give them 10 cards, pick two at random, show me one and I'll guess 66% correctly whether it's the higher of the two cards), I would seriously hope that they would figure out the trick without me needing to guess very much, if at all. If they were smart, they'd shut up when they figured it out and would keep pulling, say 9 and 10, and showing me the 9 all the time.

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