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 = [0.52711170160166, -40.1317830052742, 153.020073134484, -
        2247.99398218827, -0.193993484669048, -1.40467557893768, 
        42.6799878114024, 0.752810643416743, 22.6657238616417, -
        622.873556909932, -0.660823667935396, 0.841267087271658, -
        25.3717501764397, 485.708963532948, 880.531517490555, 
        2650155.92794626, -0.359287150025783, -656.991567673753, 
        2.41768149185367, 0.856873461222588, 0.655143675313458, -
        0.213535213206406, 0.00562974957606348, -316955725450471.0, -
        0.000699997000152457, 0.0119845803210767, 1.93848122022095e-05, -
        2.15095749182309e-05]
    Pr = P / 100
    sigma = s / 5.3
    suma = 0
    for i, j, ni in zip(I, J, n):
        suma += ni * (Pr + 0.76) ** i * (sigma - 0.818) ** j
    return 860 * suma
