100/30= 10/3 person-days

Best arrangement, every 3 days:

Three people don't go to work for two days, 17 people go to work.

1 day, 4 people are not working, 16 people are working.

Every three days, 10 people are absent from work. In this way, 100 people will not go to work in 30 days. At least 16 people are working every day for the whole month.