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Answer
Success!
Decimal probability = 0.000240096

↓Steps Explained:↓

In a standard 5-card poker hand for 1 deck:

Calculate P(4 of a kind)

4 of a kind Hands

4 of a kind

Calculate Total 5 Card hands

Choose 5 cards from 52 cards

Total Hands  =  52!
  (52-5)! * 5!

Total Hands  =  52!
  47! * 5!

Total Hands  =  (52 * 51 * 50 * 49 * 48) * 47!
  47! * (5 * 4 * 3 * 2 * 1)

Cancelling the 47!, we get:

Total Hands  =  311,875,200
  120

Total Hands = 2,598,960

Build Probability

Possible 4 of a kinds = Possible ways to get 4 cards of one kind and any 5th card

Possible 4 of a kinds = 13 different 4 of a kind choices * 48 remaining 5th cards = 624

Probability of a 4 of a kind  =  Possible 4 of a kind hands
  Total Hands

Reduce top and bottom by 624

GCF = Greatest Common Factor

P(4 of a kind)  =  624
  2,598,960

GCF for 1 and 4165 = 624

P(4 of a kind)  =  1
  4,165

Final Answer

Decimal probability = 0.000240096
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