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Answer
Success!
Decimal probability = 1.38517E-5

↓Steps Explained:↓

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

Calculate P(Straight Flush)

Straight Flush Hands

Straight Flush

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 Straight Flushes = Possible ways to get 5 cards in a row starting with A-9 card

Possible Straight Flushes = 9 ways * 4 possible suits (Spade,Heart,Diamond,Club) = 36

P(Straight Flush)  =  Straight Flushes
  Total Hands

Reduce top and bottom by 12

GCF = Greatest Common Factor

P(Straight Flush)  =  36
  2,598,960

GCF for 3 and 216580 = 12

P(Straight Flush)  =  3
  216,580

Final Answer

Decimal probability = 1.38517E-5
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