I recently returned from the O’Reilly Open Source Convention in Portland Oregon. In the final talk of the event, Paul Fenwick, always an amazing presenter, offered a puzzle during his talk that I thought I’d share.

I will offer the puzzle below, but the best way to experience it is to watch Paul’s talk, Mindware Upgrades For Fun And Profit.  No special technical or other background is necessary to enjoy it.

Albert is looking at Betty.  Betty is looking at Charlie.  Albert is a computer programmer.  Charlie is not.  Is there a computer programmer looking at a non-computer programmer?

What was most interesting to me is that the audience of about a thousand really smart people is then invited to raise their hands for “Yes” “No” or “Not Enough Information” and all but fewer than 10 people raised their hand for the same wrong answer!  I failed along with the vast majority, and the other wrong answer outweighed the right answer (which might have had zero votes… I don’t remember.)

If no one posts the right answer here after a while, I’ll come back with an explanation, but I do encourage you to go watch Paul explain.

A friend posed this to me today:

Devise an alternate set of denominations for coins and bank notes requiring a minimum number of denominations and such that any amount from $0.01 to $100 could be paid with four units of currency.

I’ve looked at this from another perspective before after it was discussed on Freakonomics but that has to do with minimizing the average number of coins needed and doesn’t go too much into the math.

What’s remarkable is that my friend nearly instantly came to an upper bound that isn’t huge and after hours five of us have failed to improve upon it.  I’ll avoid stating that bound here for now, but will come back and add it to the comments in a bit if there is no traction.

 Another interesting tidbit is that unlike U.S. denominations where a greedy algorithm can verify a minimal number of units that are required (take as many of the largest as you can, then as many of the next largest, etc.) denominations where the denominations aren’t multiples of each other don’t allow this.  Making 15 in 12-5-1 shouldn’t be 12-1-1-1 — it should be 5-5-5.  Even doing verification requires dynamic programming solutions.