[tex]C^k_n = \dfrac{n!}{k!(n-k)!} \Rightarrow C^4_7=\dfrac{7!}{4!(7-4)!}\\C^4_7=\dfrac{7!}{4! \cdot 3! } = \dfrac{7 \cdot 6 \cdot 5 \cdot 4!}{4! \cdot 3 \cdot 2 \cdot 1 }^{\big (\big 4\big !} = \dfrac{7 \cdot 6 \cdot 5}{6}=7 \cdot 5 = \boxed{35}[/tex]
