CPM terminology: Slack (total float) represents the difference between which pair of schedule times for an activity?

Introduction / Context:
In CPM networks, float (slack) quantifies scheduling flexibility. Correctly defining slack is crucial for identifying activities that can be delayed without affecting the project completion date.

Given Data / Assumptions:

• An activity has earliest start (ES), earliest finish (EF), latest start (LS), latest finish (LF).
• Activity duration is fixed (deterministic CPM).
• Total float is used (not free or independent float).

Concept / Approach:
Total float is the amount of time an activity can be delayed without delaying the project finish date. Numerically, total float = LS − ES = LF − EF. This is the difference between the latest allowable time and the earliest time for the same endpoint (start or finish). Option (b) captures this relationship in words.

Step-by-Step Solution:

Verification / Alternative check:
If TF = 0, the activity is critical; any delay increases project duration. If TF > 0, the activity may slip within TF without impact.

Why Other Options Are Wrong:
Options (a), (c), and (d) misuse terms like “normal expected time”; slack does not compare to cost-estimating durations, it compares earliest and latest schedule times.

Common Pitfalls:
Confusing free float (LF of predecessors to ES of successors) with total float; mixing start and finish times inconsistently.

Final Answer:
Latest allowable time and earliest completion time