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359 logic/weighing/gummy.bears.p




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This article is from the Puzzles FAQ, by Chris Cole chris@questrel.questrel.com and Matthew Daly mwdaly@pobox.com with numerous contributions by others.

359 logic/weighing/gummy.bears.p


Real gummy drop bears have a mass of 10 grams, while imitation gummy
drop bears have a mass of 9 grams. Spike has 7 cartons of gummy drop bears,
4 of which contain real gummy drop bears, the others imitation.
Using a scale only once and the minimum number of gummy drop bears, how
can Spike determine which cartons contain real gummy drop bears?

logic/weighing/gummy.bears.s

Spike uses 51 gummy drop bears: from the 7 boxes he takes respectively
0, 1, 2, 4, 7, 13, and 24 bears.

The notion is that each box of imitation bears will subtract its
number of bears from the total "ideal" weight of 510 grams (1 gram of
missing weight per bear), so Spike weighs the bears, subtracts the
result from 510 to obtain a number N, and finds the unique combination
of 3 numbers from the above list (since there are 3 "imitation" boxes)
that sum to N.

The trick is for the sums of all triples selected from the set S of
numbers of bears to be unique. To accomplish this, I put numbers into
S one at a time in ascending order, starting with the obvious choice,
0. (Why is this obvious? If I'd started with k > 0, then I could
have improved on the resulting solution by subtracting k from each
number) Each new number obviously had to be greater than any previous,
because otherwise sums are not unique, but also the sums it made when
paired with any previous number had to be distinct from all previous
pairs (otherwise when this pair is combined with a third number you
can't distinguish it from the other pair)--except for the last box,
where we can ignore this point. And most obviously all the new
triples had to be distinct from any old triples; it was easy to find
what the new triples were by adding the newest number to each old sum
of pairs.

Now, in case you're curious, the possible weight deficits and their
unique decompositions are:

3       = 0 + 1 + 2
5       = 0 + 1 + 4
6       = 0 + 2 + 4
7       = 1 + 2 + 4
8       = 0 + 1 + 7
9       = 0 + 2 + 7
10      = 1 + 2 + 7
11      = 0 + 4 + 7
12      = 1 + 4 + 7
13      = 2 + 4 + 7
14      = 0 + 1 + 13
15      = 0 + 2 + 13
16      = 1 + 2 + 13
17      = 0 + 4 + 13
18      = 1 + 4 + 13
19      = 2 + 4 + 13
20      = 0 + 7 + 13
21      = 1 + 7 + 13
22      = 2 + 7 + 13
24      = 4 + 7 + 13
25      = 0 + 1 + 24
26      = 0 + 2 + 24
27      = 1 + 2 + 24
28      = 0 + 4 + 24
29      = 1 + 4 + 24
30      = 2 + 4 + 24
31      = 0 + 7 + 24
32      = 1 + 7 + 24
33      = 2 + 7 + 24
35      = 4 + 7 + 24
37      = 0 + 13 + 24
38      = 1 + 13 + 24
39      = 2 + 13 + 24
41      = 4 + 13 + 24
44      = 7 + 13 + 24

Note that there had to be (7 choose 3) distinct values; they end up
ranging from 3 to 44 inclusive with 7 numbers missing: 4, 23, 34, 36,
40, 42, and 43.

-- David Karr (karr@cs.cornell.edu)

 

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