LS and LF question from pmzilla

Submitted by darenatto on Fri, 03/06/2009 - 20:52

Activity X has an FS relationship with a 1-day lag with succeeding Activity Y. Activity X also has a SS relationship with a 5-day lag with succeeding Activity Z. Assume that Activity X has no other immediate succeeding relationships. Given the following: Activity Y: Late Start is Day 11; Late Finish is Day 15 Activity Z: Late Start is Day 11; Late Finish is Day 15 Activity X: Duration is 5 days. Calculate Activity X's Late Start and Finish dates.( Assume: Duration = finish date minus start date + 1)

Can someone please explain the logic behind this?

A:Late Start is Day 5; Late Finish is Day 9.
B:Late Start is Day 6; Late Finish is Day 11.
C:Late Start is Day 6; Late Finish is Day 10.
D:Late Start is Day 5; Late Finish is Day 11.

Correct answer is A.But I think it would be B.Any suggestions pls?

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mirrar

Mon, 03/30/2009 - 09:26

May you please explain how you got answer B?

Answer B will give Activity X a duration of 6... (11-6+1)

So the answer should be either A or C.

My logic is:

Since Activity X and Activity Y are having FS relationship with a lag of 1 day, that means the Late Finish of Activity X has to be 9. (If there is no 1 day lag, then LF of Activity X would be 10). This makes A the only answer.

 

crabapple

Mon, 03/30/2009 - 12:40

When you perform a backward pass for the activity X, LF has to be 11  ..?

Since,  Activity X's LF will Activity Y's and Activity Z's LS.

I am following the solution per Rita book.

My solution was B as well.

-crabs

 

jenshrimp

Tue, 04/07/2009 - 21:27

The S-S Relationship means that Z cannot start until X has Started (+ 5 Days of Lag). It does not mean that Z starts immediately after X starts + 5 days or concurrently with X + 5 days.

 

The only lag you would worry about in this instance is the Finish to Start relationship of X and Y. Since the LS of Y is Day 11, the Lag day between X and Y is Day 10.  Therefore, the LF of X is 9, and the LS is 5.

 Hope this helps.

mirrar

Wed, 04/08/2009 - 02:15

I guess I found the key to this question now.

Both answer A and B can be correct, as there are two methods of finding LS/LF/ES/EF:

1. Rita's method:

Duration=EF-ES or LF-LS, successor ES (or LS)=predecessor EF (or LF)

2. Non-Rita method:

Duration=EF-ES+1 or LF-LS+1, successor ES (or LS)=predecessor EF (or LF)+1

However, since the question has stated an assumption: Duration = Finish date - Start Date + 1, non-Rita method is to be used. Hence A is the correct answer for this question.