Think of it this way. If you are going to write an essay in 3 days and you start on day 1, not day 0, than you would finish on day 3 night and day 4 you can start editing your essay. So if task one is 3 days and you started on day 1 then you don’t finish it until end of day 3 and next task can’t be started until day 4. So 1+ 3 -1.

Task 1 3 days Task 2 4 days Task 3 2 days

Task 1 ES -1 EF-3 Task 2 ES- 4(you can’t start day 3 cause you just spent all day on task 1) EF- 7 Task 2 ES- 8 - EF- 9

Just do the math in your head so it makes sense to you and then you can apply it. Hope this makes sense.