While it is permissible for an employer and an employee to agree upon different base rates of pay for different types of work, it is settled under the Act that where a rate has been agreed upon as applicable to a particular type of work the parties cannot lawfully agree that the rate for that work shall be lower merely because the work is performed during the statutory overtime hours, or during a week in which statutory overtime is worked. Since a lower rate cannot lawfully be set for overtime hours it is obvious that the parties cannot lawfully agree that the working time will not be paid for at all. An agreement that only the first 8 hours of work on any days or only the hours worked between certain fixed hours of the day or only the first 40 hours of any week will be counted as working time will clearly fail of its evasive purpose. An announcement by the employer that no overtime work will be permitted, or that overtime work will not be compensated unless authorized in advance, will not impair the employee's right to compensation for work which he is actually suffered or permitted to perform.