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Robert Kohr says:
"Combination locks are great because it means that you don't have to worry about losing the key, but if you don't remember the lock combo, you are in trouble."
The solution to this problem is to write an encrypted version of the combo directly on the lock itself using a sharpie, and then all you have to do is work back from the encrypted version if you forget the lock combination. This can be as simple as adding your birthday to the number, and when you need to recover the number, you just subtract your birthday from it.Link