def x_aver_top_mass(xp_mass, xpf_mass):
    """
    Calculates the average mass concentration at the top of column.
    Parameters
    ----------
    xp_mass : float
        The mass concentration  of distilliat, [kg/kg]
    xpf_mass : float
        The mass concentration of point of feed, [kg/kg]  
    Returns
    -------
    x_aver_top_mass : float
        The average mass concentration at top of column, [kg/kg]
    References
    ----------
    Дытнерский, стр.230, формула 6.8
    """            
    return (xp_mass + xpf_mass) / 2