Time: PDM calculate logical relationships

Activity 1 has a duration of 20 days, activity 2 of 10 days, activity 3 of 5 days and activity 4 of 6 days.

What is the minimum total duration between the milestones A and B?

  36 days
  37 days
  39 days
  42 days

I thought it is 36, but that's wrong. How do you calculate that?

20 (Act1) + 10-2(Act3) + 3+6(Act4) = 37
Act 2 will be not added as activity 3 is finishing before 2 days of it and act 4 is starting on finishing of 3.

Can someone pls explain the relationship from Act2 - Act4:

 from Act2 -> Act3 = FF-2 means 30-2 = 28, right?

(i) What will cause Act2 to finish on the 28th day when actually Act2 takes 10 days = 30th day?

(ii) What will be the maximum total duration btw Milestones A and B under same situation?

Thanks