4 Keys and 4 Locks

There are 4 keys and 4 locks. What is the maximum number of times you need to try the locks so as to match all 4 keys to their locks?


6 tries are needed.

If you try 3 keys on the 1st lock, you will know that the 4th key is a match. So, you only need a maximum of 3 tries for the 1st lock.

Similarly, the 2nd lock needs a maximum of 2 tries and the 3rd lock needs only 1 try. Thus, a maximum of 6 (3 + 2 + 1) tries are needed.

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