This regular rate was then applicable to the first 40 hours regularly worked . . . and the overtime rate (150% of the regular rate) became effective as to all hours worked in excess of 40.Walling v. Helmerich & Payne, Inc., 323 U.S. 37, 40-41 (1944) (citation and footnote omitted); see also Walling v. Youngerman-Reynolds Hardwood Co., 325 U.S. 419, 425 (1945) (“the determination of the regular rate [is] a matter of mathematical computation, the result of which is unaffected by any designation of a contrary ‘regular rate’ in the wage contracts”). Because the adjusted rate of $25.