# [C (clang)], 71 bytes

<!-- language-all: lang-c -->

    float f(a,n,p,r)float a,n,p,r;{for(r=1;p--;)r*=(a-p)/(a-p/n);return r;}

[Try it online!][TIO-kn1r7hs4]

[C (clang)]: http://clang.llvm.org/
[TIO-kn1r7hs4]: https://tio.run/##fVLRjpswEHzPV6wiRYKcaWwMBkTTl6pf0USVS8wVNTWWTdWoUX691MaGXi6nol2L2VmG8dpN0py5fB7H9tzzAdqII4kU0rHHAdXXtteR3pNaJUkd6@0@4omKd27dybjWYvipJej6Nv7gnYzi1XUF9unkAIMwwxf@@Qh7uKYIfOQYAb2PlD0mTee81feC0gsSBEuk/43Sx2sd5XXwrOCMLcC6yl5A4lw65KCjaVDzsxIXJZpBnB6d@cDvSrcsilOpKjGtUpKzkqVFlk81QjNSUMwymhc0K8vpM4xJRRhmtk5YldO0mlqLosgwJYyVxJap78QkI7SqMmYTl1bd1YPX5hvXW7uK5rvQ3ur6cPmUHi7VR5v5GsFLTNf3e1SmO/fSfkVEUgSm1xC5gXbyJC6WwnV4fQ@m@y36NppHE@9CYbtUanh6mrpj8HdmPh5ulcLdmfhjfUfLmZZv0mqm1QPtd2Jsg7vtIBGo@DUrLLuc55v63EwK/KuJTCJiu9cwm399StvONlpvTgh8QvIBNi1szEHaOYefIzBoORCnewx2bqvb@Kdpz/zZjMmvvw "C (clang) – Try It Online"   

Calculates the given formula. Returns a `float`.