The international commission consists of nine people. The commission's materials are stored in a safe. How many locks should the safe have, how many keys for them need to be made, and how should they be distributed among the commission members so that access to the safe is possible only when at least six members of the commission are present?
According to the condition, any five people cannot open the safe. This means they do not have the key to a certain lock. At the same time, any other member of the commission must have this key. Therefore, we need to install $C_{9}^{5}=C_{9}^{4}$ locks. Four keys to each lock are given to a certain quartet of commission members, and different keys are given to different quartets.

## Answer

126 locks, with 4 keys to each lock.