Consider the following timeline of page references: 9 6 2 3 4 4 4 4 3 4 4 2 5…

Concept

At reference time t, the working set W(t, Δ) is the set of distinct pages appearing in the most recent Δ page references ending at t.

Repeated references to the same page count once because a working set records membership, not frequency.

Application

1. The penultimate reference is the 25th value, 2; the final value 7 is outside the requested instant.

2. The five-reference window ending at the 25th value is 3, 3, 9, 6, 2.

3. Removing the repeated 3 leaves the distinct-page set {2, 3, 6, 9}.

Cross-check

Moving the window one reference forward to the final value would produce 3, 9, 6, 2, 7 and would include page 7. Its absence at the penultimate instant confirms the boundary used above. Therefore, the working set is {2, 3, 6, 9}.