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

    .. math::
        R = \\frac{t}{kA}

    Parameters
    ----------
    k : float
        Thermal conductivity of a substance [W/m/K]
    t : float
        Thickness of the substance for a given value of R, [m]
    A : float, optional
        Area; normally 1, [m^2]

    Returns
    -------
    R : float
        Thermal resistance of a substance [K/W]

    Examples
    --------
    >>> k_to_R(k=0.5, t=0.025)
    0.05

    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 / (k * A)
