Need Help - Question on network diagram--From Oliver Lehman's sample 75 question

 Hi,

Can someone please explain me the  answer to the below question. This question is from Oliver Lehmans 75 sample 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.

Between activity 2 and 3, there is FF -2 Days.Between activity 3 and 4, there is FS +3 Days

all activities are sequential i.e. Start-activity1-activity2->activity3->activity4->end

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

1.36 days

2.37 days

3.39 days

4.42 days

Thanks,
Neil

 

20+10+3+6=39

Given  Answer is 37. I am not sure how....

Correct answer is 37

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Answer is 37 because it is like this

20 (activity 1) + 10 (activity 2) + 7 (activity 4) = 37

Here is how.

The relationship between a1 and a2 is straight forward so you take 20.

The relatioship between a2 and a3 is a little tricky and that is FF - 2 which means there is a lag of 2 days which further means a3 will finish 2 days before a2 ends. So we can say a3 will end on day 8 of a2. Since a2 has the bigger duration here so we can simply take 10 here.

After that comes a4. As you know there is FF + 3 relationship between a3 and a4 which means a4 will start 3 days after a3 ends. Since a4 will take 6 days hence we simply add 6 + 3 to come to result 9. But there is a little thing you are forgetting here. Since a3 ended on day 8 of a2 which is 2 days before a2 ended hence a4 will start right after that. So we can simply deduct 2 from 9 to come up with result 7.

Now add all results 20 + 10 + 7 = 37