def _Backward3b_T_Ps(P, s):
    """Backward equation for region 3b, T=f(P,s)

    Parameters
    ----------
    P : float
        Pressure [MPa]
    s : float
        Specific entropy [kJ/kgK]

    Returns
    -------
    T : float
        Temperature [K]

    References
    ----------
    IAPWS, Revised Supplementary Release on Backward Equations for the
    Functions T(p,h), v(p,h) and T(p,s), v(p,s) for Region 3 of the IAPWS
    Industrial Formulation 1997 for the Thermodynamic Properties of Water and
    Steam, http://www.iapws.org/relguide/Supp-Tv%28ph,ps%293-2014.pdf, Eq 7

    Examples
    --------
    >>> _Backward3b_T_Ps(20,5)
    640.1176443
    >>> _Backward3b_T_Ps(100,5)
    847.4332825
    """
    I = [-12, -12, -12, -12, -8, -8, -8, -6, -6, -6, -5, -5, -5, -5, -5, -4,
         -3, -3, -2, 0, 2, 3, 4, 5, 6, 8, 12, 14]
    J = [1, 3, 4, 7, 0, 1, 3, 0, 2, 4, 0, 1, 2, 4, 6, 12, 1, 6, 2, 0, 1, 1, 0,
         24, 0, 3, 1, 2]
    n = [0.527111701601660, -0.401317830052742e2, 0.153020073134484e3,
         -0.224799398218827e4, -0.193993484669048, -0.140467557893768e1,
         0.426799878114024e2, 0.752810643416743, 0.226657238616417e2,
         -0.622873556909932e3, -0.660823667935396, 0.841267087271658,
         -0.253717501764397e2, 0.485708963532948e3, 0.880531517490555e3,
         0.265015592794626e7, -0.359287150025783, -0.656991567673753e3,
         0.241768149185367e1, 0.856873461222588, 0.655143675313458,
         -0.213535213206406, 0.562974957606348e-2, -0.316955725450471e15,
         -0.699997000152457e-3, 0.119845803210767e-1, 0.193848122022095e-4,
         -0.215095749182309e-4]

    Pr = P/100
    sigma = s/5.3
    suma = 0
    for i, j, ni in zip(I, J, n):
        suma += ni * (Pr+0.760)**i * (sigma-0.818)**j
    return 860*suma