Where an employee in the same workweek performs work which is exempt from the overtime requirements of the Act under section 13(b)(9), and also engages in work to which the overtime requirements apply, he is not exempt from overtime provisions of the Act in that week. (See McComb v. Puerto Rico Tobacco Marketing Co-op Ass'n., 80 F. Supp. 953, affirmed, 181 F. 2d 697; Mitchell v. Hunt, 263 F. 2d 913; Abram v. San Joaquin Cotton Oil Co., 46 F. Supp. 969; McComb v. del Valle, 80 F. Supp. 945; Walling v. Peacock Corp., 58 F. Supp. 880.) As explained in § 793.13, work which does not come within the occupational duties of an announcer, news editor, or chief engineer, or which is not related and incidental thereto, is not exempt work under section 13(b)(9). The mere isolated or occasional performance of insubstantial amounts of such nonexempt work will not defeat the exemption for the employee. Where, however, an employee, in a particular workweek, performs a substantial amount of nonexempt work to which the overtime provisions of the Act are applicable, the employee is not exempt under section 13(b)(9) in that workweek. For administrative purposes an employee who spends 20 percent or more of the hours he works in a workweek in such nonexempt work, will not be considered exempt under section 13(b)(9) in that workweek.