import math


def boil_time_factor(boil_time_minutes):
    """Boil time factor for hop utilization.

    Parameters
    ----------
     boil_time_minutes : float
        Amount of time hops spend in the boil, in minutes.

    Returns
    -------
     boil_time_factor : float
        Multiplicative factor for hop utilization based on boil time.

    """
    return (1 - math.exp(-0.04 * boil_time_minutes)) / 4.15