Semaine Européenne
Concours de mathématiques

The answer from Montargis

We make two groups of six weights.

We divide one of the group into two groups of three. If the weight is the same, we take the other group of six. In this group, we take a group of three weights, then, we weigh X1 with X2. If it is the same, we weigh X1 or X2 with X3. If it is the same, we take the other group of three weights. Then, we weigh X4 with X5, if it is the same, we weigh X4 or X5 with X6.

And X6 will be heavier or lighter than X4 or X5.

The smallest possible number of weighings is between 3 and 5.