IMO Shortlist 1988 problem 11

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Dodao/la: arhiva
2. travnja 2012.
The lock of a safe consists of 3 wheels, each of which may be set in 8 different ways positions. Due to a defect in the safe mechanism the door will open if any two of the three wheels are in the correct position. What is the smallest number of combinations which must be tried if one is to guarantee being able to open the safe (assuming the right combination is not known)?
Izvor: Međunarodna matematička olimpijada, shortlist 1988