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Project Management (project network diagram) Question Watch

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    The question is in the linked image, together with its solution that was given during class.
    http://i47.tinypic.com/2jbmtn8.jpg
    To all you project management experts or fellow IT students out there - some help will be greatly appreciated!
    My question is with regards to the Latest Finish for B being 20 while the Latest start for D being 18. Why is this correct or rather, my question is - does it matter if 20 ends after 18? I'm aware that since D's constraint, "Start After B has started" means that D is technically not affected by B's Latest Finish. I just wanted a confirmation because in most other project network examples, the activity before the end activity always had its Latest Finish = Earliest Finish of end activity.
    My second question is why is the free float for B = 4? Have I misunderstood the formula for free float? Isn't the formula for free float:
    (EF of the next activity - EF of that activity) = free float ? Or are there exceptions, and if so how does one identify them?
    Also, if the free float of B is 4, then the LS is 18, which makes its LF 24 - this date ends after the end activity's LF. (D is the end activity right?) Does this not matter? Because if it doesn't matter then why did was B's initial LF of 24 cancelled to an LF of 20? Just very confused with these questions.
    Thank you so much for your time guys.
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    Crikey, the flow diagram is horrendously and unnecessarily complicated in my view.

    Much prefer the activity on a line diagram.

    As far as I can see, the shortest path is A-C-D which makes the critical path 20 days duration.

    Performing a backwards path from the finish shows float for activity B is 2 days.

    I cannot see how the latest finish for B is 20 days nor get a float of 4 days from the constraints.
 
 
 
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