Problem Solving (PS) | Re: Suppose a “Secret Pair” number is a four-digit number in which two adj

Good question.

Let's assume the pair of equal digits in in the first 2 cases, hence 0 is not usable.

We'll have 9*1*9*8 options, which equals 648 numbers.

Now, if the pair is something like ABBC -> 9*9*1*8 (since now we can use 0 in the middle)

If the pair is in the end, which will give a number like ABCC: 9*9*8*1

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