def get_operand_string(mean, std_dev):
    """
    Method to get operand string for Fsl Maths
    
    Parameters
    ----------
    mean : string
        path to img containing mean
    std_dev : string
        path to img containing standard deviation
    
    Returns
    ------
    op_string : string
        operand string
    """
    
    str1 = "-sub %f -div %f" % (float(mean), float(std_dev))
    op_string = str1 + " -mas %s"
    return op_string