A poker formula question

[eqgh5uea]

here is how you figure this out ...

52 cards in a deck
4 cards known (ten ten , ten nine)
48 cards left in the deck preflop (52-4)
4 "good" cards (1 ten, 3 nine)

first card of the flop has a 4 out of 48 chance

now there is one less "good" card and one less card in the deck

second card has a 3 out of 47 chance

now there is one less "good" card and one less card in the deck

third card has a 2 out of 46 chance

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(4/48) * (3/47) * (2/46)

That explains it.