def rho_top_liq(x_aver_top_mass, rho_lc_x_aver_top, rho_hc_x_aver_top):
    """
    Calculates the destiny of liquid at top of column.
    Parameters
    ----------
    x_aver_top_mass : float
        The average mass concentration at top of column, [kg/kg]
    rho_lc_x_aver_top : float
        The destiny of low-boilling component for mass average concentration at the top of column, [kg/m**3]
    rho_hc_x_aver_top : float
        The destiny of high-boilling component for mass average concentration at the top of column, [kg/m**3] 
    Returns
    -------
    rho_top_liq : float
        The destiny of liquid at top of column, [kg/m**3]
    References
    ----------
    Романков, стр.12, формула 1.3
    """            
    return ((x_aver_top_mass / rho_lc_x_aver_top) + ((1 - x_aver_top_mass) / rho_hc_x_aver_top))