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Programming Enigma Puzzles

26 February 2018

Posted by on **From New Scientist #2229, 11th March 2000** [link]

In cross-country matches, teams consist of six runners. The team scores are decided by adding together the finishing positions of the first four runners to finish in each team. The team with the lowest score is the winner. Individuals never tie for any position and neither do teams because if two teams have the same score the winner is the team with the better last scoring runner.

The fifth and sixth runners to finish in each team do not score. However if they finish ahead of scoring runners in another team they make they make the scoring positions of those scoring runners, and the corresponding team score, that much worse.

In a recent match between two teams, I was a non-scorer in the winning team. Each team’s score was a prime number, and if I told you what each team’s score was you could deduce with certainty the individual positions of the runners in each team. I won’t tell you those scores, but if you knew my position you could, with the information given above, again deduce with certainty the individual positions of the runners in each team.

(1) What was my position?

(2) What were the positions of the scoring runners in my team?

[enigma1073]

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This Python program runs in 90ms.

Run:[ @repl.it ]Solution:(1) You finished 8th; (2) The scoring runners in your team finished 2nd, 4th, 5th, 6th.The positions for the winning (setters) team were:

giving a total score of 17 (prime).

The positions for the losing team were:

giving a total score of 23 (prime).

There are thirteen ways in which the scores 17 and 23 can be made up as the sum of four integers each, with no duplication; four of those ways do not use 8. So I don’t see how either piece of information could determine the positions uniquely. Please explain!

@Hugh: You might want to check again.

You can’t have, for instance, team A = (1, 2, 4, 10) = 17 and team B = (3, 5, 6, 9) = 23, with (7, 8, 11, 12) being the non-scoring runners. As if runner 10 scored for A, then team A’s non-scoring runners would be 11 and 12, so B’s runners would be (3, 5, 6, 7, 8, 9), so their scoring runners would by (3, 5, 6, 7) not (3, 5, 6, 9).

In fact there is only one possibility for A=17 and B=23.

Similarly there are two other A, B scores that only have one possible assignment of positions.

Then looking at the non-scoring runners for team A for each of these three scenarios, we find that there is a position that only appears in one of the possibilities, and that gives us the required answer.

Thanks, Jim. It was clear that 11 and 12 could not be scorers, but I didn’t think it through far enough.