# [C (gcc)], 66 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);a=r;}

[Try it online!][TIO-kn1rlwgc]

[C (gcc)]: https://gcc.gnu.org/
[TIO-kn1rlwgc]: https://tio.run/##fVLRjtsgEHzPV6wiRTI53ICxsS03fan6FU104hx8tZpiZCw1apRfbwoG@y6JrtYueHaHYVmo49e6vl6bYycGaCKBFda4Rx4HVJ2bro/6La10HFeoX28jEWu0ceNGoUps@@py/SVaFaHFeQH28@sHaYZn8X0PWzgnGLxlBAO7tYQ/Oksmv1T3kspLUgyzJf@1wtujkvZKZNJwxc3AVpa@g9RV6pCDLs1u9ORJy3qQh8favJFPhRtmxTFUFoSVCc14wZM8zcYYZSnNGeEpy3KWFsW4jBBaUk64jVNeZiwpR2qe5ylhlPOC2jDzTEJTysoy5dZJYdVdPNRa/xD92o6y/il7X@pyd/qW7E7lV@vZEsN7zJa3Z9SmPXbKrqIyzkOm6yFq1QCtOsiTTZEq/H4G0/6RXRNNrUGbEFjPkQqenkY2Av9y3jYTViu8oJGxr@4IaiKoDwh6IugPCMYS3KMHhUGj@6y02flW7wXcibUwo4J4MZGJJbInDh164@neMptouTpg8A7xF1g1sDI7ZbvtWodEmFWYNQaD51ty2@xDdZfF5fq3bo7i1Vzj3/8A "C (gcc) – Try It Online"   

Calculates the given formula. Returns a `float`.