29 C.F.R. § 784.116

Where an employee, during any workweek, performs work that is exempt under section 13(a)(5) or 13(b)(4), and also performs nonexempt work, some part of which is covered by the Act, the exemption will be deemed inapplicable unless the time spent in performing nonexempt work during that week is not substantial in amount. For enforcement purposes, nonexempt work will be considered substantial in amount if more than 20 percent of the time worked by the employee in a given workweek is devoted to such work (see Mitchell v. Stinson, 217 F. 2d 210). Where exempt and nonexempt work is performed during a workweek by an employee and is not or cannot be segregated so as to permit separate measurement of the time spent in each, the employee will not be exempt (see Tobin v. Blue Channel Corp., 198 F. 2d 245; Walling v. Public Quick Freezing and Cold Storage Co., 62 F. Supp. 924).