The questions no 31 and questions 27 below are from www.oliverlehmann.com 75 free questions. pls can someone , anyone explain how they arrived at their answers. Thanks so much

(Question 31) 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

--------------------------------------------------------------------------------

27. A project manager made 3-point estimates on a critical path and found the following results:

Assuming ±3 sigma precision level for each estimate, what is the standard deviation of the allover path?

App. 4.2 days
App. 5.2 days
App. 6.2 days
You can not derive the path standard deviation from the information given.