def R_to_k(R, t, A=1.0):
    """Returns the thermal conductivity of a substance given its thickness
    and thermal resistance.

    .. math::
        k = \\frac{t}{RA}

    Parameters
    ----------
    R : float
        Thermal resistance of a substance, (K/W) if A is 1 m^2, otherwise must
        be [m^2*K/W]
    t : float
        Thickness of the substance used in the measurement of R, [m]
    A : float, optional
        Area; normally 1, [m^2]

    Returns
    -------
    k : float
        Thermal conductivity of a substance [W/m/K]

    Examples
    --------
    >>> R_to_k(R=0.05, t=0.025)
    0.5

    Notes
    -----
    When solving problems of changing areas, this value may be calculated with
    an area other than 1 m^2. Values in tables reported as properties of
    materials are often divided by area already; the conversion holds if A is 1.

    References
    ----------
    .. [1] Bergman, Theodore L., Adrienne S. Lavine, Frank P. Incropera, and
       David P. DeWitt. Introduction to Heat Transfer. 6E. Hoboken, NJ:
       Wiley, 2011.
    """
    return t / (A * R)
