Respuesta :

[tex]_n C_r = \frac{n!}{r!(n-r)!}[/tex]
[tex]_n C _r[/tex] -- the number of combinations of r objects in total n objects.

So for r=6, n=20 we have 
[tex]_n C_r = \frac{20!}{6!(20-6)!} = 38760[/tex]

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