ME and THIS LOCK

I have a 4 digit lock for my locker. One day, I changed the code and accidentally 5 minutes later I forgot the code. All I ever knew is that I never used 2 same digits for the code. For example : I never used 5504 because there are 2 5's there. When I tried to open the lock, what is the probability I will succeed?? Express \(\frac{a}{b}\) as \(a+b\).

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