def attenuation(originalGravity: float, finalGravity: float) ->float:
    """
    Determines Attenuation of yeast.

    Parameters
    ----------
    originalGravity : float
        Original gravity of wort.
    finalGravity : float
        Final gravity of beer.

    Returns
    -------
    float or None
        Attenuation.
    """
    if originalGravity > finalGravity:
        return (originalGravity - finalGravity) / (originalGravity - 1)
    else:
        return
