Chords

How many 4-tones chords (chord = at the same time sounding different tones) is possible to play within 7 tones?

Correct result:

n =  35

Solution:

n=C4(7)=(74)=7!4!(7−4)!=7⋅6⋅53⋅2⋅1=35n = C_{{ 4}}(7) = \dbinom{ 7}{ 4} = \dfrac{ 7! }{ 4!(7-4)!} = \dfrac{ 7 \cdot 6 \cdot 5 } { 3 \cdot 2 \cdot 1 } = 35

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