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Chests (Posted on 2024-03-20) Difficulty: 3 of 5
You are in a room with 2N Plaminians and C locked chests. One of the chests contains a treasure. Half of the Plaminians are consecrated, and know which chest contains the treasure. The other Plaminians do not know which chest has the treasure. All of the Plaminians know which ones are consecrated. You do not know which Plaminians are consecrated, or which chest has the treasure.

The Plaminians have agreed that you may ask each one a yes/no question, and they will answer truthfully if they know the answer, and randomly if they do not know.

What sequence of questions will give you the greatest chance of locating the chest with the treasure? For what values of N and C is success guaranteed?

No Solution Yet Submitted by K Sengupta    
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Some Thoughts simulation | Comment 3 of 4 |
There are problems with the comments I previously gave.

I ran a simulation for asking successive Plaminians. If any at any stage is known to be consecrated, ask that one if a particular one of the chests you hadn't asked about before contains the treasure. But if there's not yet a known consecrated individual, ask anyone who hasn't been asked a question before if a certain individual of unknown consecration status is in fact consecrated.

If, after all have been questioned, none has been identified by a consecrated Plaminian as containing the treasure, a random choice is made among those not identified as not containing the treasure.

10,000 trials are done for each N and C combination.


clearvars
for c=1:20
  for n=1:10
    succ=0;
    for tr=1:10000
      asked= false(1,2*n);
      cons= zeros(1,2*n);
      consecrated=cons;
      consecrated(randperm(2*n,n))=1;
      chest=zeros(1,c);
      treas=randi(c);
      chest(treas)=1;
      done=false;
      while ~done
        idx=find(cons==1 & asked==false);
        if ~isempty(idx)
          idx2=find(chest>-1);
          if chest(idx2(1))==1;
            succ=succ+1;
            done=true;
          else
            asked(idx(1))=true;
            chest(idx2(1))=-1;
          end
        else
          idx=find(asked==false);
          if isempty(idx)
            if randi(c-sum(chest==-1))==1
              succ=succ+1;
            end
            break
          end
          idx2=find(cons==0);
          idx3=find(idx2~=idx(1));
          if ~isempty(idx3)
            asked(idx(1))=true;
            if consecrated(idx3(1))==0
              cons(idx3(1))=-1;
            else
              cons(idx3(1))=1;
            end
          else
            if randi(c-sum(chest==-1))==1
              succ=succ+1;
            end            
            done=true;
          end
        end
      end
    end
    fprintf('%5d ',succ)
  end
  fprintf('\n')
end

The number of successful finding of the treasure per 10,000 tries:

      N
    \   1     1     1     1     1     1     1     1     1     1
  C
  1   10000 10000 10000 10000 10000 10000 10000 10000 10000 10000 
  2    7528  7576  7502  7561  7491  7501  7529  7620  7487  7452 
  3    5038  5027  4952  5045  5022  5074  5020  5089  5066  4944 
  4    3790  3696  3778  3679  3757  3787  3739  3773  3792  3788 
  5    3053  3068  2955  2917  2952  3062  3067  2901  2992  2964 
  6    2378  2466  2456  2524  2460  2445  2486  2539  2511  2435 
  7    2176  2137  2057  2127  2179  2162  2088  2151  2137  2163 
  8    1895  1828  1850  1929  1856  1888  1882  1846  1928  1911 
  9    1751  1702  1666  1656  1649  1628  1654  1692  1681  1720 
 10    1503  1457  1508  1545  1503  1438  1499  1425  1568  1523 
 11    1396  1323  1330  1352  1365  1345  1406  1367  1414  1340 
 12    1280  1254  1267  1225  1198  1208  1243  1275  1251  1227 
 13    1159  1213  1131  1121  1140  1155  1165  1210  1193  1137 
 14    1085  1028  1059  1073  1082   987  1077  1065  1034  1059 
 15     961  1044   926  1015   985   959   967  1016  1021   984 
 16     893   899   920   926   914   892   905   925   895   962 
 17     891   916   868   873   851   936   891   891   883   829 
 18     831   794   758   860   834   834   810   834   787   809 
 19     774   814   791   807   809   796   751   741   833   737 
 20     737   713   731   786   717   767   766   773   754   759 

  Posted by Charlie on 2024-03-20 13:24:49
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