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Worked Examples · Example 4
Q.

A small project has the following activities, with their durations (in days) and immediate predecessors:

ActivityPredecessor(s)Duration (days)
A—4
B—3
CA2
DB5
EC, D3

Draw the logical structure of the network (describe it), find the earliest and latest event times, and identify the critical path and the project's minimum completion time.

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Step 1 — Set up the network's events. Event 1 (start) → A and B both begin here (no predecessors). A (4 days) finishes at event 2; C (predecessor A) runs from event 2 to event 3 (2 days). B (3 days) finishes at event 2 as well only if it shares a tail with A — but since A and B have no shared predecessor relationship stated, and D depends on B while C depends on A, event 1 is the common start for both A and B, with A leading to the event where C starts, and B leading to the event where D starts; C and D both feed into event 4, from which E (3 days) runs to the final event 5.

Step 2 — Forward pass (earliest event times, EE). E1=0E_1=0 (start). E2=E1+4=4E_2 = E_1+4=4 (after A). Also treat B's completion event similarly: EE at B's finish =E1+3=3=E_1+3=3. CC runs from A's finish, duration 2: reaches E=4+2=6E=4+2=6. DD runs from B's finish, duration 5: reaches E=3+5=8E=3+5=8. Both C and D feed into the event before E, so that event's earliest time is the larger of the two incoming paths: E4=max⁡(6,8)=8E_4 = \max(6,8)=8. Finally E5=E4+3=8+3=11E_5 = E_4+3 = 8+3=11 (project's earliest finish).

Step 3 — Identify the two paths from start to finish explicitly. Path A→C→E: duration 4+2+3=94+2+3=9 days. Path B→D→E: duration 3+5+3=113+5+3=11 days.

Step 4 — Backward pass (latest event times, LL), starting from L5=E5=11L_5=E_5=11. Working back through E (duration 3): the event before E must have L=11−3=8L=11-3=8. Working back along D (duration 5) from that event: B's finish event needs L=8−5=3L=8-5=3. Working back along C (duration 2) from the same shared event: A's finish event needs L=8−2=6L=8-2=6. …

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