Enigmatic Code

Programming Enigma Puzzles

Enigma 1418: Non-scoring runners

1 Comment Posted by on 24 June 2013

From New Scientist #2578, 18th November 2006

In cross-country races, teams consist of six runners; team scores are calculated by adding together the finishing positions of the first four runners to finish in each team, the team with the lowest score winning. Individuals never tie. The fifth and sixth runners in each team do not score but if they finish ahead of scoring runners in other teams they make the positions of those runner and their teams’ scores worse.

In a race involving three teams, each of six runners, no two teams had the same score. The third team’s score was a multiple of the second team’s score, which itself was a multiple of the winning team’s score. The sum of the positions of the first three finishers in the winning team, the sum of the positions of the first two finishers in the second team and the position of the first finisher in the third team were identical.

What were the positions of the non-scoring runners in the second team?

[enigma1418]

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One response to “Enigma 1418: Non-scoring runners”

  1. Jim Randell 24 June 2013 at 8:42 am

    This Python program runs in 107ms.

    from itertools import combinations, product
from collections import defaultdict
from enigma import irange, diff, printf

# there are 18 runners, so 18 finishing positions
pos = list(irange(1, 18))

# accumulate scores by positions
r = defaultdict(list)
for t in combinations(pos, 4):
  r[sum(t[:4])].append(t)

# C is a multiple of B, B is a multiple of A
# so the maximum value of A is the max score / 4
(mn, mx) = (min(r.keys()), max(r.keys()))
for A in irange(mn, mx // 4):
  if A not in r: continue
  for B in irange(2 * A, mx // 2, A):
    if B not in r: continue
    for C in irange(2 * B, mx, B):
      if C not in r: continue

      # choose non-intersecting solutions
      for (a, b, c) in product(r[A], r[B], r[C]):
        s = set(a).union(b, c)
        if len(s) != 12: continue
        # check the conditions
        if not(c[0] == sum(b[:2]) == sum(a[:3])): continue

        # find the non-scoring runners
        ns = diff(pos, s)
        for cn in combinations((x for x in ns if x > max(c)), 2):
          for bn in combinations((x for x in ns if x > max(b) and x not in cn), 2):
            an = tuple(x for x in ns if x > max(a) and x not in bn and x not in cn)
            if len(an) != 2: continue

            printf("A={A} {a} {an}, B={B} {b} {bn}, C={C} {c} {cn}")

    Solution: The non-scoring runners in the second team were placed 12th and 14th.

    Reply

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