Le Monde puzzle [#14.2]

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I received at last my weekend edition of Le Monde and hence the solution proposed by the authors (Cohen and Busser) to the puzzle #14. They obtain a strategy that only requires at most 19 steps. The idea is to start with a first test, which gives a reference score S0, and then work on groups of four questions, whose answers can be found in at most three steps. For instance, starting with x1,…,x25, the second test uses (1-x1),(1-x2),x3,…,x25, and changes the score S0 by -2,2 or 0. In the first two cases, this determines y1,y2 and it suffices to use x1,x2,(1-x3),(1-x4),x5…,x25, to find y3,y4 in one or two steps. If the score S0 does not change, considering x1,(1-x2),(1-x3),(1-x4),x5…,x25, and then maybe x1,(1-x2),x3,(1-x4),x5…,x25, produces again the value of y1,…,y4. If one repeats the algorithm one group of four after another, there are six such groups and the maximal number of step is


since the final answer y25 is known by deduction from S0. An additional improvement not mentioned in the journal is achieved in checking after any change whether or not the new score is equal to zero. (In both my solution and theirs, there is an extra step to propose the correct solution, which means in my case an exact average of steps equal to 25, by the geometric argument.)

For the current solution, here is an R code that evaluates the distribution of the number of steps:

for (t in 1:10^5){

for (j in 1:6){



if (abs(Delta-Delta0)==2){


The fit by a binomial is rather poor, but this is not surprising given the two-stage decision. In any case, this does better than my earlier solution!

Filed under: Books, R, University life Tagged: Le Monde, mathematical puzzle

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