Royal Flush frequency - Texas Hold 'Em

[NeoGeo]

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If you hold two cards from a royal flush, the odds of hitting it by river are C(45,2)/C(50,5). This is about 0.0467%.

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I understand the logic of C(45,2), but I am not sure of why C(50,5). Can you show me the logic how it derives to royal count. It is quite intereting. Thank you.