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Answer
Success!
Decimal probability = 0.0475390156

↓Steps Explained:↓

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

Calculate P(2 pair)

2 pair Hands

2-pair

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 2 pairs = 13 cards to choose from in combinations of 2

Possible ways to choose ranking of 2 pairs = 13!/(2! * 11!)

Possible ways to choose ranking of 2 pairs = (13 * 12)/2 = 78

Possible ways to choose 2 combos of 4 cards = [4!/(2! * 2!)]2 = [(4 * 3)/2]2 = 36

36 Possible choices * 78 2 pair combos * 44 5th cards = 2,808 * 44 5th cards

Possible 3 of a kinds = 123552

Probability of 2 pair  =  Possible 2 pair hands
  Total Hands

Reduce top and bottom by 624

GCF = Greatest Common Factor

P(2 pair)  =  123,552
  2,598,960

GCF for 198 and 4165 = 624

P(2 pair)  =  198
  4,165

Final Answer

Decimal probability = 0.0475390156
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