Is there a formula for calculating working time?

Question

Mathematics

I can not formulate a question for Google, how to call "time windows", so I ask for your help:
Let's say a person works exactly 8 hours, respectively, 16 hours of "rest". There is a time for the task, which is, for example, 40 hours. When working 8 hours a day, this will take 5 days, but not 120 hours (5 * 24), but 104 - five times 8 hours and 4 times 16 (8 + 16 + 8 + 16 + 8 + 16 + 8 + 16 +8).
Work for 8 hours will take, it turns out, not "one day", a multiple of days, but 8 hours, while 9 working hours will take 25 actual hours.
Actually, I started to reinvent the wheel, taking into account an even and an odd number, with if-else blocks ... well, I thought that maybe there is a normal mathematical solution "in one line".
Everything is suitable as an example - a description in words, links to Wikipedia, examples in PL (I understand php and js).

Answer 1

For the ideal case - work every day strictly for 8 hours - the solution is the simplest.
But for real calculations, this, of course, is not suitable, if only because of weekends, holidays and postponements of holidays.