How many combinations are possible in this question

combinations

I was curious how many combinations are possible in this scenario:

  • The characters allowed to make up the combination are so:
    • Letters a-z (lowercase only)
    • Numbers 0-9
    • Underscores _
    • Periods .
  • The combination is 4 digits
  • The combination cannot end with a period (.)

Just a side note, this can have repeating characters.

I also appreciate it a lot if someone were to answer this.

Best Answer

We have $26+10+1+1=38$ possible characters. We want $4$ characters and the combination cannot end with a period so we have $38^3 \times 37$ possible combinations.

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